infn interning 18.qxp layout 1 -...

[#newphysics]

year 10, number 18 / 2015

asimmetriehalf-yearly magazine of the Italian

National Institute for Nuclear Physics

INFN InternING 18.qxp_Layout 1 05/11/15 18:19 Pagina 1

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asimmetrie

asimmetrie 18 / 2015 / #newphysics

Dear Readers,

The discovery of the Higgs boson came at the end of a longjourney, where the direction was marked, but the road had tobe built. Fifty years of efforts and, thanks to a series ofinnovative technologies and a clear vision, at last came theLHC accelerator and the long awaited discovery.So what now? Now we are looking out onto a world we don’tknow, where there are no paths, and even the directions to befollowed aren’t very clear. Sure, we know there is somethingout there, indeed, there is rather a lot. There is dark matter,with its very evident gravitational effects. Some peoplemaintain that the extinction of the dinosaurs was due to itseffects. The speed at which the universe is expanding, whichincreases with the passage of billions of years and that we, inthe absence of any explanation, attribute to a mysterious darkenergy, is another enigma to be solved. In the world ofconventional particles, instead, the neutrino with its uncertainnature and mass, is waiting to be placed in a model otherthan the standard one according to which it was attributedvarious properties.And what shall we do to investigate this unknown world?Which direction should we take and which paths should webuild? Research into dark matter relies on the future of theLHC at even higher energy, on capturing its interactions in thesilence of the underground detectors, and on the signalsobserved by the satellites that scan the universe. The study ofdark energy, rather less advanced, is entrusted to terrestrialtelescopes and satellites, which will at least be able to tell uswhether this continuous acceleration has always been presentor if it varies in time. To study neutrinos, highly refinedexperiments are under construction and will operate awayfrom the disturbance of cosmic rays, for example, in thedepths of the Gran Sasso (a mountain in Italy, ed.), or inmines and observatories under the sea or ice.We are using a successful and sustainable combination oftechnology with patience and method to make progress inscience.

Happy Reading.

Fernando FerroniINFN President


asimmetrieMagazine of the Italian NationalInstitute for Nuclear Physics (INFN)

Half-yearly, year 10,number 18, April 2015English edition: November 2015

Managing DirectorFernando Ferroni, presidente Infn

Scientific Committee DirectorEgidio Longo

Scientific CommitteeVincenzo BaroneMassimo PietroniGiorgio RiccobeneBarbara Sciascia

Editor-in-ChiefCatia Peduto

Editorial staffEleonora CossiVincenzo Napolano Francesca Scianitti Antonella Varaschin

Francesca Cuicchio(infografica)

with the collaboration ofGuido Altarelli, Luca Amendola, ElisaBernardini, Gianfranco Bertone, BrunaBertucci, Marco Ciuchini, Daniele DelRe, Giuseppe Giuliani, Carlo Giunti,Mark Levinson, Andrea Romanino,Marco Serone, Kip Thorne

English translation Alltrad srl

English revisionJohn Walsh

Editorial office contactsINFN Communications Office Piazza dei Caprettari 70 I-00186 Rome T +39 06 6868162F +39 06 68307944 [email protected] www.infn.it

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Rome Court Registration number 435/2005dated 8 November 2005. Magazine publishedby INFN.

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We apologize if, for reasons beyond our control,any sources have been wrongly cited or omitted.

3asimmetrie 18 / 2015 / #newphysics


as 18 / 4.15[#newphysics]

Today is already tomorrow 5by Guido Altarelli

Up and down the scales 11by Marco Ciuchini

The world starting with an “s” 14by Andrea Romanino

Desperately seeking SUSY 16by Daniele Del Re

[as] with other eyes 18Particle fever.by Mark Levinson

A good push 20by Lucio Rossi

[as] reflexes 24Magnets for the future.by Eleonora Cossi

Behind the scenes of the universe 25by Gianfranco Bertone

News from the space station 27by Bruna Bertucci

… that moves the sun and other stars 31by Luca Amendola

Elusive mysteries 34by Carlo Giunti

On the rocks 38by Elisa Bernardini

Fundamental chords 42by Marco Serone

[as] intersections 44They are us.by Kip Thorne

[as] roots 46The new, a hundred years ago.by Giuseppe Giuliani

[as] illuminations 48Just a click away.



Today is already tomorrowPhysics after the Higgs boson

by Guido Altarelli

5


After the first phase of the Large HadronCollider (LHC) experiments at CERN, particlephysics is faced with a new paradox. On onehand, the theory of strong and electroweakinteractions, the so-called standard model, hasachieved some outstanding successes, likethe discovery of the Higgs boson, passingprecision tests of every type, etc. On the other,strong theoretical reasons (mainly the questionof hierarchy or naturalness) and evidentexperimental problems (for example the natureof dark matter) suggest that the standardmodel theory is not entirely complete and thatsigns of new physics should actually alreadyhave emerged from the experiments in theaccelerators. Paradoxes are always veryinteresting, because solving them can lead toa real turning point in the development of newtheories in physics.Completed by the Higgs boson, the standardmodel is a perfectly functional theory: it is infact re-normalisable, which means that, after afinite number of experimental measurements,the theory is well “calibrated” and can, inprinciple, be used to predict the result of anyother measurement, without ambiguity up tothe highest energies. As far as our universe is

concerned, if the standard model is deemed tobe entirely valid, the universe is meta-stable: itcould collapse into a new and completelydifferent state. Fortunately, the likelihood ofsomething like that happening in the timecomparable to the age of the universe ispractically zero.Given that decades of experiments inaccelerators have always reaffirmed thevalidity of the standard model, why should wecontinue to invoke new physics? Above all,because we have some very impressiveexperimental evidence “from the sky”, orrather from astrophysics and cosmology:almost the entire density of energy in theuniverse is vacuum energy (around 73%),which can be described in terms of a non-nullcosmological constant called dark energy (seep. 31, ed.) and dark matter (approx. 22%, seep. 25, ed.), neither of which is included in thestandard model. Furthermore, the standardmodel does not account for the properties ofneutrinos (masses and mixings) (see p. 34,ed.) that, as already established, have a non-zero mass (at least two of them), which ishowever much smaller than that of quarks andcharged leptons. Besides, the standard model

a.On 4th July 2012, Rolf Heuer(Director of CERN), Fabiola Gianotti(then project leader of the ATLASexperiment, now Director designateof CERN) and Joe Incandela (projectleader of the CMS experiment atthat time) announced the discoveryof the Higgs boson, contributing tothe success of the standard modelof particles.



7


cannot explain baryogenesis, that is the predominance ofmatter over antimatter in the universe (see p. 27, ed.). Then there are conceptual issues that “call upon” new physics.These include the fact that the gravitational interaction (gravityin layman’s terms) is outside the scope of the standard modeland that the question of the quantum theory of gravitationremains unsolved (string theory could produce a solution, seep. 42, ed.).The quantum effects of gravitation are only appreciable atmassively higher energies than those explored in the LHC (inthe region of 10 TeV, or 104 GeV) and thus on the so calledPlanck scale, in the region of 1019 GeV. Extrapolating theknown theories to slightly smaller scales that are, nonetheless,still well beyond current experimental limits, provides somevery interesting clues about the behaviour of new physics. Infact, the (calculable) evolution of the gauge coupling constants(the parameters that determine the intensity of the differentfundamental interactions) as a function of the energy scaleindicates that these tend to assume the same value on thescale of grand unification (1015 GeV): this behaviour ispredicted by the Grand Unification Theories, abbreviated asGUT, according to which the three interactions that exist at the“low” energies studied in the LHC (strong, weak andelectromagnetic) are based upon a single fundamentalinteraction on a very high energy scale (see fig. c, p. 9).Unification on the same scale is however only approximated inthe standard model and could also lead to a prediction forproton decay that is too fast with respect to existingexperimental limits. Both of these problems could be overcomeif we had a new physics, or rather a theory that goes beyondthe standard model, on energy scales below that of the grandunification.The solution to many of the problems with the standard modelcan be found in a more fundamental theory (in which thestandard model is the limit of low energy, see p. 11, ed.),perhaps in a theory on the Planck scale, which also includesthe gravitational interaction in unification. For example, thesolution to the flavour problem, i.e., the explanation of the threegenerations of quarks and leptons and the mysterious hierarchybetween their masses might not be accessible at low energy.There are questions with the standard model that require (suchas the hierarchy problem) or suggest (such as dark matter) a

solution in proximity to the electroweak scale. The hierarchyproblem consists of the fact that, in the standard model, thevalue of the Higgs boson’s mass depends heavily on the valuesof the masses of any new particles that exist on higher scales.In other words, this hierarchy of masses means that themeasured value of the mass of the Higgs boson (equal to 125GeV) is the consequence of a cancellation of terms that couldbe much higher, due to coupling between the Higgs boson andother new particles. For this mechanism to be “natural”, thesenew particles must not be too heavy, in that the terms inducedby their coupling with the Higgs boson must not be much higherthan the observed value of the latter’s mass. Thus, in order toextrapolate the theory up to the scale of grand unification orthe Planck scale, without having to “invoke” an extremelyaccurate and un-natural “fine tuning” of these cancellations,we need to expand the theory and introduce new particles thatmay have masses not far from the electroweak scale. Themost studied theoretical method is that of supersymmetry (orSUSY, from SUper SYmmetry), a symmetry between bosonsand fermions, according to which every known particle in thestandard model is associated with a correspondingsupersymmetrical partner or s-partner (see page 14, ed.).The dependency on high scales cancels out for higher energiesof the masses of the supersymmetric particles (or at leastsome of them) and this expanded standard model (for examplelike the so-called minimal supersymmetric standard model) isinsensitive to the presence of even heavier particles, thusovercoming the hierarchy problem. Furthermore, ifsupersymmetry were an exact symmetry, the s-partners wouldhave exactly the same mass as the corresponding particles ofthe standard model. However, since none of these new objectshas ever been observed, we have to conclude that they have agreater mass, which means that supersymmetry must only bean approximate symmetry or, in jargon, a broken symmetry.However, to solve the hierarchy problem and satisfy the needfor “naturalness”, the s-partners must not be too much heavierthan the particles of the standard model. That is why manywere expecting new physics on the scale of 1 TeV, andtherefore accessible by the LHC (and even by LEP2, the LargeElectron-Positron collider in operation at CERN until 2000).Today, data obtained both from research into new particles andthe exploration of their possible virtual effects require a high


b.The boxes in this figure similar to those in “snakes andladders” represent the different energy scales (in blue) andthe corresponding distance scales (in orange). Thecorrespondence between energy, expressed in electron volts,and distance, expressed in metres, is shown by the equationE(eV) ~ 2 x10-7/L(m), since small distances correspond tohigh energies and vice-versa. The figure shows the masses ofthe particles of the standard model already known (electrons,muons, tau, different types of quarks, neutrinos, Higgsboson, etc.), those of other as yet hypothetical particles(axions, sterile neutrinos...) and the energies characteristic ofcertain physical phenomena observed or attainable in particleaccelerators.


level of fine-tuning. Nature doesn’t seem tocare much about our idea of “naturalness”!Supersymmetry has many strengths apart fromthose of a purely theoretical nature: it correctsthe unification of gauge coupling on a scale ofabout 1016 GeV, it guarantees sufficientstability for protons and has ideal dark mattercandidates, specifically neutralinos. Neutralinosare special WIMPs (Weakly Interacting MassiveParticles, meaning ‘heavy particles thatinteract weakly’). They are, in fact, particleswith a mass of between a few GeV and a fewTeV, they are stable (or do not decay) and theirinteractions are such as to ensure that they areproduced in the primordial universe with theabundance required by cosmologicalobservations. WIMPs are the subject of intenseresearch, not only in the LHC, but also inexperiments in laboratories and in space,beneath the sea and under the ice.Solving the problem of dark matter, whichsuggests the existence of new particles withmasses of less than the TeV, is a crucial issuefor today’s particle physicists.Nobody has even the slightest idea of therelevant mass interval: this ranges from axions(hypothetical bosons without electric chargeand that interact rather little with matter), witha mass of around 10-5 eV, to sterile neutrinos(not subjected to any interaction betweenthose present in the standard model) with amass of a few keV, to WIMPs and other moreexotic candidates too.

For other phenomena that point to newphysics there are, instead, some veryplausible extensions of the standard model.For example, the minuscule masses ofneutrinos can be elegantly explained by the so-called see-saw mechanism (see p. 34, ed.).Where does the community of physicists standon the “naturalness” issue? Many think theproblem might go away at least in part, if themuch invoked new physics decides to appearin the second phase of the LHC at 13-14 TeV,which has just started. Many models havebeen developed where new physics is actuallyclose-by, but with characteristics that have sofar made it invisible. Others contemplate moreexotic scenarios: dependency on the highmasses we have already talked about woulddo no harm, if there were no particles up tothe Planck scale and if a mechanism due tothe unknown theory of gravity could resolve theproblem of naturalness at those high energies.In this case though, the questions of darkmatter, the masses of neutrinos andbaryogenesis would all have to be resolved byphysics around the electroweak scale. This ispossible by assuming the existence of newlight sterile neutrinos (one of a few KeV andtwo others of a few GeV).Other physicists inspired by the question of thecosmological constant see an “anthropic”solution to the problem, in which the values ofcertain parameters are explained not on thebasis of theoretical predictions, but from the

c.The coupling constants of thestrong, weak and electromagneticinteractions as a function of theenergy scale according to thestandard model (left) and accordingto the minimal supersymmetricstandard model (right). In thesupersymmetric model, the threecoupling constants assume thesame value at an energy scale ofaround 1016 GeV, as envisaged inthe grand unification scenarios.



observation that different values would giverise to a universe that would be completelydifferent from the one observed. In the case ofthe cosmological constant, the question of“naturalness” consists of the fact that this ismany orders of magnitude smaller than wouldbe expected on the basis of theoreticalestimates. However, the observed value isclose to the maximum that is possible in orderto permit the formation of galaxies and thus

our very existence. Therefore if our universe isjust one of the many that are continuouslybeing produced by the vacuum throughquantum fluctuations, as predicted by somecosmological theories, each of which containsa different physics, it might be that we live in avery exceptional universe, which is ratherunlikely from a theoretical perspective, but theonly “anthropic” one, i.e. the only one wherewe are able to exist.

d.A small part of the 27 km-longLHC accelerator tunnel, justoutside Geneva on the Swiss-French border, where protonbeams started circulating againon 4th April 2015.


BiographyGuido Altarelli has been professor emeritus of theoretical physics at the University ofRoma Tre. He was Director of the Rome Section of the INFN from 1985 to 1987.Between 1987 and 2006 he worked at the Theory Division of CERN, and was TheoryDivision Leader from 2000 to 2004. He passed away on the 30th of September 2015.


Up and down the scalesThe role of effective theories

by Marco Ciuchini

11


Physics is characterised by the presence ofscales: scales of distance, energy, time, speedor any other physical quantity, fundamentalscales or scales related to a specific problem.Interesting physical phenomena occur at verydifferent scales: galaxy clusters collide atdistances in the region of 1022 metres, thecollision of two quarks in an LHC takes placeat around 10-19 metres.Were a physical phenomenon to depend onwhat happens at all scales in the same way, itwould be really very difficult to describe.Fortunately (albeit with some notableexceptions) the separation of scales applies:as a general rule, physical phenomenaoccurring at a given scale are not affected bythe details of physics associated with verydifferent scales and in many cases largephysical quantities change when passing fromone scale to another. That is why, when thereare several scales at play, it is convenient touse a non-exact theory which, however,describes the relevant physics at a given scalein the most appropriate way. This is whatphysicists call an effective theory, as opposedto the fundamental theory which is ideally validat all scales.Let us imagine, for example, that we want tocalculate the path of a ball on a billiard table.We think we can use a fundamental theory,namely relativistic mechanics based onEinstein’s special theory of relativity. We couldcertainly use it, but we don’t usually: why?Because the problem involves two verydifferent scales: the typical velocity of the ball,let’s say of around 1 m/s, and the speed oflight c = 3×109 m/s, which is a fundamentalconstant of the theory. It is more convenient tointroduce an approximate theory, developed byconsidering the velocity of the ball as adistortion of the limiting case considering thespeed of light as infinite. The effective theorydeveloped using the first adjustment at thelimiting case is nothing more than Newtonian(classical) mechanics, the theory we allstudied at school, since this, though anapproximation, is the most appropriate andconvenient theory for describing objectsmoving at speeds much smaller than thespeed of light, i.e. all the objects of which wehave direct experience. In the field of elementary particle physics (bothrelativistic and quantum), all physical scalesare based on energy scales (which explainswhy particle physicists always use multiples ofthe electron-volt). The fundamental theorymust thus describe physics at all energiesincluding all the elementary particles thatexist, even those we have yet to discover!

As mentioned earlier, effective theories can,on the other hand, be conveniently used todescribe low-energy processes. Yes, but lowcompared to what? Some energy scales areassociated with fundamental interactions: thescale of strong interactions (or QCD scale),about 1 GeV; the Fermi scale, in the region of100 GeV, related to the masses of W and Zbosons, mediators of weak interactions; thePlanck scale, equal to 1019 GeV, related to theuniversal gravity constant. Whenever weconsider processes with typical energies thatare much smaller than one of thesefundamental scales (there are others too, suchas those for the masses of heavy fermions),we can define an effective theory obtained,like in the example of classic and relativisticmechanics, as a distortion at the limiting casein which we consider the energy at the highestscale in the theory as infinite. This theory onlycontains the relevant particles at the scale ofthe processes being considered, while theheaviest particles are generally uncoupled.Furthermore, if we also know the theory at thehigh scale, we will be able to calculate theadjustments at the limit case to arbitrarily highprecision. The effective theory can still be veryuseful even without knowing the theory at thehighest scale: it allows us to parameterise ourignorance in terms of a finite number ofconstants and so as to respect the knownproperties of physics at the low scale.To date, the standard model, the benchmarktheory for particle physics, is in accordancewith all observed phenomena. Is it afundamental theory or is it the firstapproximation of an effective theory,applicable at the energies reached by ouraccelerators, say, at the Fermi scale (althoughthe LHC has now started exploring the TeVscale, 10 times bigger)? The standard modeldoes not contain gravity (because we have yetto learn how to include it!), so something mustdefinitely change at the Planck scale, wherethe gravitational interaction is no longernegligible. It is therefore natural to think of thestandard model as an effective theoryobtained by considering the Planck mass asinfinite. There are, however, several theoreticaland observational reasons (dark matter, toname but one) that suggest there might be anintermediate energy scale between the Fermiscale and the Planck scale, associated withthe mass of new heavy particles. This scale,called the scale of new physics, and theseparticles, not included in the standard model,are the new physics we are looking for.The problem of the scale of new physics is thatwe think it exists, but we don’t know for



certain its magnitude: if it is less than the energy that can bereached in the LHC, we will soon be able to observe the newparticles and test whether and with which of the extensions ofthe standard model developed over the last 40 years(supersymmetry, again to name just one) they are compatible.Once we have discovered the theory of new physics, thestandard model will continue to be valid as an effective theoryfor processes on the Fermi scale, and we will also be able tocalculate the corrections due to the presence of the newparticles. If instead, we do not succeed in producing newparticles, and have to wait for a higher energy accelerator (seep. 20, ed.), we can try to see if there are any deviations from thepredictions of the standard model due to the presence of newheavy particles. Indeed, Heisenberg’s uncertainty principlestates that particles make a virtual contribution to physicalprocesses at a certain energy level, even if they are too heavy tobe produced. In this game, the effective theory can be used intwo ways: adopting an approach from the high scale to the lowscale, attempting to “guess” the theory according to the scale of

new physics, typically on the basis of some new symmetry, andusing this to calculate the corrections to the “standard model”effective theory, or adopting the opposite approach, from the lowto the high scale, using experimental data to try to identify thepossible corrections to the standard model in the hope that thisinformation will lead to the new theory.Which method is more useful? The standard model wasproposed in the 1960s using both approaches: it is based onsymmetry, which however was identified using an effectivetheory, called the V-A theory, used in previous years to describeweak interactions. Vice versa, the Higgs mechanism forspontaneous symmetry breaking, which is also an essentialingredient for constructing a realistic model, was introducedwithout any true observational stimulus, by analogy withproblems of the physics of condensed matter and only now,following the discovery of the Higgs boson, can we start to useeffective theories to describe its couplings. We know fromexperience that we must use all the methods available to us inour search for new physics!

a.The equation on the cup describesthe standard model.

BiographyMarco Ciuchini is a theoretical physicist, director of research and head of the INFN Roma Tre division.He studies the phenomenology of elementary particles and is mainly concerned with quark flavourphysics.

Web links

http://en.wikipedia.org/wiki/Effective_field_theory

http://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/

http://ocw.mit.edu/courses/physics/8-851-effective-field-theory-spring-2013/




There are valid reasons to believe that new phenomena, newparticles, new principles, that would give us an even deeperunderstanding of nature, await us beyond the current frontier ofthe standard model, in a region of energies that have neverbeen reached and that now, after decades of preparation, theLHC has started to explore. Many consider supersymmetry tobe the most plausible conjecture about where theseexplorations might lead and one of the most promising ideasfor new physics beyond the standard model.The symmetry concept has played a crucial role in thedevelopment of fundamental physics. Think, for example, of thespacetime symmetry that forms the basis of the theory ofrelativity, the interpretation of fundamental forces in terms ofsymmetry groups or the idea of spontaneous symmetry breakingthat underlies the discovery of the Higgs boson; nature reallydoes seem to show a partiality for symmetry. Supersymmetry isa new kind of symmetry. Unlike the known symmetries, itconnects particles with very different characteristics andbehaviours, bosons and fermions. Examples of fermions are theparticles that constitute known matter, examples of bosons arethe Higgs particle, and those which transmit forces (photons,gluons and W± and Z bosons), and they differ in their angularmomentum (spin): the first have half-integer spin, the latter havea full integer spin. If this symmetry exists in nature, for everyboson there would be an as yet unknown matching fermion andvice versa (see fig. a): supersymmetric partners or s-partners.The supersymmetric partner of a fermion is denominated byadding the prefix “s” to the name of the corresponding fermion,while the supersymmetric partner of a boson is called by the

name of the matching boson plus the suffix “ino”. So, forexample, the supersymmetric partner of the electron is calledselectron, the partner of the top quark is stop squark and thatof the Higgs boson is Higgsino. The discovery of supersymmetricpartners in the LHC would be the resounding confirmation of thesupersymmetry theory.There are many reasons why we believe this might happen. Theproblem of naturalness, or hierarchy, requires the existence ofnew particles, presumably visible in the LHC, that explain whythe energy scale that characterises the standard model (100GeV) resists the pressure of enormous corrections according towhich it would be far higher. The particles envisaged bysupersymmetry would provide a particularly convincingexplanation.Considered as a whole, supersymmetry is an internallycoherent and technically very sound theory which, in principle,allows the behaviour of the laws of nature to be extrapolated upto the Planck scale (1019 GeV).The solution to some of the problems that require new physicsbeyond the standard model, such as the prevalence of matter

The world starting with an “s”Supersymmetric extensions of the standard model

by Andrea Romaninoa.The particles of the standard model(divided into bosons and fermions)and their respective s-partners.


over antimatter in the known universe or the origin of the massof neutrinos, may lie in those energies, not distant from aunified theory of unknown forces. Furthermore, the predictionsof supersymmetry correspond perfectly with those of thetheories of the grand unification, according to which at leastthree of the four fundamental forces of nature(electromagnetic, weak and strong force) might simply bedifferent aspects of a single unified force. This latter idea isparticularly intriguing not only for its elegance and the deepunderstanding it fosters, again, in terms of symmetry, but alsobecause it allows us to understand in a single stroke thequantitative characteristics of the forces between the particlesdescribed by the standard model, and it is hard to imagine thatsuch a result is fortuitous. Moreover, as already stated, it isfully confirmed by the theory of supersymmetry, in which theintensity of the three interactions according to the standardmodel can be extrapolated to precisely the same value at theenergy of the grand unification scale, i.e., in the region of 1016

GeV, another result that is difficult to ignore. Lastly, one aspectof supersymmetry that is much appreciated is the possibility ofexplaining the nature of dark matter (see p. 25, ed.). Theexperimental coherence of the theory would in fact result in thestability of one of the supersymmetric partners, which in thatcase would pervade our universe and could indeed constituteits dark matter.Thus, there are clearly many convincing arguments to supportthe theory of supersymmetry. Not surprisingly, after thediscovery of the Higgs boson, supersymmetry has ended up atthe top of the list of particle physicists’ “wanted” items. Itmust be said that during the LHC’s first run (from 2008 to2013) no supersymmetric particles were caught, and no othercandidates emerged that could solve the problem ofnaturalness. This has led some theorists to question the

validity of naturalness as the basis for believing that newparticles will be discovered in the LHC. Although suchconsiderations are premature, it is already clear that, were theargument for naturalness to collapse, we would find ourselvesfaced with a paradox, an explanation for which would becomethe central theme of our research. Solving this would involve aradical change of paradigm, for example, the use of anthropicconsiderations rendered possible by the multiverse hypothesissupported by some theoretical physicists (according to whom,ours is just one of the many possible parallel universes).Nonetheless, the theory of supersymmetry can still have animportant role. It may, in fact, allow us to confirm theunification of forces and explain the origin of dark matter. Itwould still be compatible with other convincing ideas aboutphysics that go beyond the standard model, such as thoseregarding the origin of masses of neutrinos. And, if the theoryof dark matter and unification are confirmed, it could lead inthe direction of possible signals in future accelerators.

BiographyAndrea Romanino is full professor at the International School forAdvanced Studies in Trieste (SISSA). He has conducted research atthe Scuola Normale Superiore in Pisa, Oxford University, CERN inGeneva and FermiLab in Chicago.

Web links

http://particleadventure.org/supersymmetry.html

http://home.web.cern.ch/about/physics/supersymmetry

b.Simulation of the production of supersymmetric particles in theLHC, as would be seen by one ofthe detectors (ATLAS). The linesrepresent the tracks left in thedetector by the particles producedin the event, especially thosederived from the immediatedisintegration of supersymmetricparticles.




Physicists have an authentic passion forconservation laws. Not only in measuring theparameters of the standard model, but also intheir search for the Higgs boson, they set strictlimits to preserve the momentum and energymeasured for the particles produced in a proton-proton collision, like those which take place in theLHC. However, when they start looking forsomething new or unexpected, their approachchanges. In fact, it is precisely the apparent non-conservation of these fundamental quantities thatmakes it possible to identify events not predictedby the standard theory. If the collision of twoprotons in the LHC also produces supersymmetricparticles, these decay and produce both standardparticles as well as stable supersymmetricparticles at the end of the cascade. These onlyinteract weakly with matter and therefore cannotbe measured. But the simultaneous presence ofparticles that are identifiable and others that arenot creates an “imbalance” that can be measured(see “in depth” on page 17). The search for supersymmetry in the ATLAS andCMS experiments is at a very advanced stage.There are tens of analyses studying the productsof proton-proton collisions. Depending on themodel of supersymmetry being tested, scientistsare looking for different types of events, such asthe presence of top or bottom quarks or of theHiggs boson. Unfortunately, in the data acquiredso far by the detectors there is no trace ofsupersymmetry. Even without producing anyconcrete signals, these tests have allowedphysicists to set limits for the mass ofsupersymmetric particles, although there areregions that have yet to be excluded. As new dataare collected, the excluded regions will no doubtincrease, unless supersymmetric particles are atlast discovered. There are scenarios in which itwould be more difficult to observe supersymmetricparticles. For example, nature could haveorganised supersymmetric particles with specificcombinations of masses, which would make itmore difficult to observe the products of theirdecay. The fact that supersymmetry has not yetbeen observed has allowed us to exclude some of

Desperately seeking SUSYLHC in search of supersymmetry

by Daniele Del Re

a.The CMS detector, at CERN Geneva,which together with ATLAS is searchingfor supersymmetry signals.


the theory’s more favoured scenarios,but not supersymmetry as a whole. Forexample, the hypothesis that considersdark matter to be made up ofsupersymmetric particles is stillperfectly plausible (see p. 25, ed.).Some searches for dark matter in theLHC look for very simple events. Forexample, in a proton-proton collision,scientists search for the presence of asingle collimated jet of high-energyparticles (a collision in which all thetracks illustrated in fig. 1b are very closeto one another, forming a type of cone)or of a single photon

or of a Z boson. Moreover, as explainedabove, the event has to be highlyimbalanced. These searches have notproduced any positive results yet either.The second phase of data collection atthe LHC, with proton-proton collisions at13-14 TeV, has just started. This newdata-taking run should be betterequipped to probe the theory ofsupersymmetry for two reasons. First,the collision energy will be almostdoubled compared to the data previouslyacquired, making it possible to moreeffectively produce any supersymmetricparticles with higher mass. This will

open the way for studying previouslyunexplored phenomena.What is more, the number of collisionsrecorded in the coming years willincrease tenfold. Since the sensitivity ofthe searches increases as morecollisions are analysed, it will bepossible to highlight small excesses dueto supersymmetry. Thus, the next fewyears will be very important for researchinto supersymmetry and new physics,and the LHC will be able to make adecisive contribution, in parallel withexperiments in space and those directlysearching for dark matter.


[as] in depth

Scales for weighing SUSY

Let us imagine a particle detector like a balance scale. The particles of thestandard model are all “visible” on the plates of the scale, because they interactwith the detectors. Some of the supersymmetric particles, instead, interact veryweakly and therefore cannot be seen. Now suppose we only put the particles ofthe standard model on the two plates. That way we can see whether the twoplates with the same weight are actually in equilibrium: this is what takes placewith standard measurements. However, if there is also a supersymmetricparticle, even though we cannot see it directly, we can deduce its presence fromthe fact that the two plates are not balanced (see fig. 1b).The LHC’s particle detectors do not actually weigh the particles, but measuretheir momentum and can verify whether the sum of the momenta of theparticles produced in a collision is balanced (this is no ordinary sum, but avectorial sum, which considers the directions of the tracks). The three drawings

in fig. 1 are simplified illustrations of the events that take place in the LHCdetectors. The event on the left (a) represents a symmetric event typical ofstandard physics. In this case, the momenta of the particles are balanced: bydividing the detector into two hemispheres, the sum of the energy deposits inthe calorimeters or of the momenta of the charged tracks between the twohemispheres is always balanced. With new physics particles, these carry a partof the momentum, but it cannot be measured if they interact weakly. Themomentum carried by the supersymmetric particles is represented by the dottedline in the figure on the right (c). The event is thus imbalanced and, as shown in(b), there is only activity in one hemisphere of the detector. Therefore thepresence of a notable imbalance unequivocally signals the presence of newphysics. Clearly, this method only works well if the supersymmetric particlecarries a significant momentum.

BiographyDaniele del Re is a university researcher at the Physics Department of the “La Sapienza” University ofRome. He has been working on the CMS experiment since 2006. He has had various roles in theexperiment and is currently coordinator of the group looking for new physics with exotic channels.

Web links

http://cms.web.cern.ch/

http://atlas.web.cern.ch/

1.Schematic illustrations of the tracks (ingrey) and of the energy deposits (inblue) recorded by the LHC experiments.The superimposed balance scalerepresents (a) the energy balance of theevent involving particles of the standardmodel only, (b) the imbalance ifunobservable supersymmetric particleswere present, and (c) the estimate ofthe energy of the supersymmetricparticle that can be obtained assumingthat the event was in effect balanced.


[as] with other eyes

Particle fever.by Mark Levinson

director of the film Particle fever

People often ask me for advice about gettinginto the film industry. I tell them, the goodnews is, you don’t need to go to film school.The bad news – I earned a PhD in theoreticalphysics. In some sense, it was perhaps thelongest pre-production preparation in filmhistory for making Particle Fever.Although the jump from physics to film mayseem like an enormous, discontinuousquantum leap, the transition actually feltremarkably organic. What entranced me aboutphysics was the profound beauty and eleganceof the theories, and the magic and mystery inthe fact that abstract symbols, devised byhumans, encoded deep truths about theuniverse. I made the transition to film when Irecognized that it was an alternate avenue forrepresenting and exploring the world that alsoseemed mysterious and magical.In the film world, I worked in the narrativefiction format. For many years, I harbored the

hope that I would find a project that couldweave together the two seemingly disparatestrands of my life and convey the excitement ofphysics in a dramatic way. I think science hasrarely been depicted well in fiction films. So Ibegan to think about various scenarios thatmight form the basis for a dramatic film. Andthen, through some potential funders for afiction script I had written, I heard about thisphysicist, David Kaplan, who wanted to make adocumentary about the start-up of the LargeHadron Collider.David was a particle theorist, actively workingin the field, and interacting with people on adaily basis who had great stakes in the LHC –some of them had been waiting for thirty yearsfor an experiment like the LHC that couldfinally tell them if any of their theories werecorrect. It was a unique moment in the historyof science, when something was going to turnon that would definitely provide answers to


a.Film director Mark Levinson in frontof the ATLAS experiment at CERN.


some of the deepest questions abouthow the universe works. And it wasimmediately clear to me that it couldprovide the perfect combination of both aprofound scientific discovery and adramatic human story.When I met David, I told him that I wasn’tinterested in making a conventional“science” documentary that would try toexplain everything. But if we could makea human, character-based story thatallowed me to use the narrativestorytelling tools I’d developed, thatwould be tremendously appealing. Andthat’s exactly what David wanted to doas well.Of course, there were many challengesto actually making the film. First, therewere 10,000 people involved in theexperiment. We always knew we wantedit to be character-based, but who shouldwe choose to follow? People were alsoscattered all over the globe. How, on arather limited budget, could we coverpeople all around the world? As a storyabout real scientific discovery, we also

constantly had to decide when and wheresomething significant might occur. Andmost pressing of all, how long should wecontinue filming? What if there was nodefinitive discovery?David and I met at the end of 2007 and Ireally started working on the film,essentially full-time, at the beginning of2008. I was constantly thinking aboutpossible endpoints, something that couldbe a satisfying end to the film. Weactually did not think they would discoverthe Higgs while we were filming. Almostall the physicists said that it would besuch a rare event and it would take theexperimentalists years to reallyunderstand their detectors. And then, aswe were trying to wrap up the film withjust tentative results, the bigannouncement came on July 4, 2012.The discovery of the Higgs became thedefinitive, fantastic end for the film.In constructing the film, the biggestchallenge was certainly making itaccessible to a non-specialist audience.We wanted the film to be intelligible to a

general audience, while at the same timeremaining authentic. The key becameknowing what to leave out. We didn’t aimfor a film that explained everything. Butwe wanted to convey the excitement, andtruth, about real scientific discovery.One of the ways I hoped to connect to amore general audience was byemphasizing the parallels betweenfrontier level scientific research and art.Many of our characters felt thisconnection. Near the end of the film,Savas Dimopoulos asks, “Why do we doscience? Why do we do art? It is thethings that are not directly necessary forsurvival that make us human”.Fabiola Gianotti provides a quote fromDante’s Divine Comedy: “fatti non fostea viver come bruti, ma per seguir virtutee canoscenza (you were not made to liveas brutes, but to follow virtue andknowledge)”. Science and knowledge arevery important, like art is very important.It’s a need of mankind. In the end, that’swhat I hope people take from ParticleFever.

b.Showing of the film Particle Feverat CERN in November 2014. Thedirector and all the “actors”,including Fabiola Gianotti (thirdfrom left), were present.




Starting from the first machines, constructed in the 1930s byJohn Cockcroft and Ernest Walton in England and by ErnestLawrence in the USA, and then the ADA collider (Frascati, early1960s), accelerators have accompanied the progress ofnuclear and particle physics right up to the discovery of theHiggs boson. They have been the main tool used by physiciststo chase the infinitely tiny, arriving at measurements as smallas 10-20 metres with the LHC at CERN in Geneva, with asensitivity a couple of orders of magnitude higher than theprevious accelerator, the LEP (the Large Electron-PositronCollider). For their capacity to concentrate energy in thesmallest of spaces, accelerators are also time machines: inthe LHC we investigate what the universe was like less thanone picosecond (10-12 seconds) after the Big Bang.The accelerated particles are generally electrons or protons, although they may also be different types of ions, ranging fromhydrogen to completely ionised uranium and even highlyunstable radioactive ions. But, for the physics of high energies,it is above all electrons, protons and muons (along with theirantiparticles) that are accelerated, although experiments withmuons are still underway.Accelerators have developed along two main lines: circularmachines (cyclotrons and synchrotrons) and linear machines

(LINAC). In both the cases energy is imparted to the particlesby means of radio frequency (RF) resonating cavities, in whichthere are electromagnetic waves.Magnets are required to guide the particle beams (for exampleto keep them in a circular orbit in the synchrotrons) and tofocus them – by magnetic lenses – to prevent them fromimpacting the walls of the vacuum chamber in which theytravel. The main components of a circular accelerator areshown in fig. a, in which the beam is made to circulate manytimes through the radio frequency cavities, which therefore donot need to have high power or be very long. The magneticpart, on the other hand, is predominant, since both themagnetic field and the accelerator radius must be increased toobtain high energies. Linear accelerators instead consist of aseries of radio frequency cavities, through each of which theaccelerated particles pass only once: high radio-frequencyvoltages (tens of millions of volts) are important in order tolimit the size of the machine. In LINACs the main purpose ofthe magnets is to keep the beam focused.The Large Hadron Collider (LHC) with its 27 km circumference,1700 huge 7 and 8 tesla superconducting magnets (by way ofcomparison, a magnetic resonance machine produces amagnetic field of one tesla and the Earth’s magnetic field,

A good pushAccelerators for the new physics

by Lucio Rossi

a.The main components of an LHC-typeparticle accelerator: radio-frequencyaccelerating cavities (a), bi-polarmagnets (b) to guide the beam andquadrupole magnets (c) to focus it.The three particle detectors (d) arealso shown


which directs the needle of a compass, is a few hundredthousandths of a tesla) and maximum energy of 14 TeV is theculmination of 40 years of work on developing superconductingcolliders, and took more than 20 years to design and build.The first step to move beyond the limits of the LHC is alreadyunderway: the High Luminosity LHC (or HILUMI) project isdeveloping new technologies for 11 and 12 teslasuperconducting magnets using a more advancedsuperconducting material that is also much more complex andcostly than the niobium-titanium (Nb-Ti) used in the LHC:niobium-3-tin (Nb3Sn). HILUMI has a limited number ofmagnets, around 50, and constitutes the ideal test bench forthis technology. HILUMI also features a new type ofsuperconducting radio frequency cavity, called a crab cavity, forrotating the beams and increasing their luminosity, i.e., thenumber of particle collisions produced by the accelerator in aunit of time.The High Luminosity LHC will be able to measure theproperties of the Higgs boson and any other new particles withgreater precision. The machine is scheduled to come online in2025. Its construction will cost around € 700 million for thecomponents alone, and it is expected to run for 10-15 years.A very different project, but which is also at the frontier ofprecision physics, is the ILC (International Linear Collider). Theadvantage of a linear collider with respect to a circular collideris that it can accelerate light charged particles like electronsand positrons, without these losing their energy when theypass through the magnets used to change their trajectory. Theenergy lost with the emission of radiation is very limited in theLINAC which makes it possible to accelerate light particles athigher energies.

Based on powerful superconducting cavities, with a voltage of30 million volts (five times higher than the LHC cavities), theaim of the ILC will be to make electrons collide with positrons.In a first phase the energy at the centre of mass (i.e., theenergy that is actually available) should be 0.5 TeV with thepossibility of reaching 1 TeV in the second phase. The project,which follows on from the German Tesla project proposed in themid ’90s, has made a lot of progress and is at an advancedstage of maturity, despite some notable technologicalchallenges. The total cost stands at around € 10 billion and itis not expected to be ready until after 2030. Offsetting theadvantage of having a technological precursor, the X-ray Fel(Free Electron Laser) project in Hamburg – a LINAC for lowerenergy electrons using the same radio frequency technologies –,the ILC project has two weaknesses: it is expensive, while notachieving the energies of the LHC even at maximumconfiguration, and, since it requires almost 50 km of hightechnology, its usefulness also critically depends on theattainable luminosity, which is much lower for LINACs than forcircular accelerators. Another electron-positron linear colliderproject is CLIC (Compact Linear Collider). Based on a system oftwo accelerators, with very high frequency copper cavities andvoltages of 100 million volts, it could, in principle, triple theefficiency of the ILC, thanks to its 3 TeV at the centre of massand 50 km in length. Promoted by CERN, with a largeinternational collaboration, it is less technologically advancedthan the “rival” ILC project. Construction could start in 2020-2025, with startup between 2035 and 2040. Its weakness isits energy consumption of around 600 megawatts (there are nosuperconducting cavities or magnets), which is equivalent to thepower generated by a medium-to-large electric power station.


b.A niobium superconducting cavityfor the ILC, enclosed in the heliumcryogenic chamber, during itsinsertion into the test station atFermiLab, in Chicago (USA).


Moreover, there is no real “scale demonstrator”: a smallprototype of the accelerator (even a few percent of the finalaccelerator) to prove that it works. However, if the LHC were todiscover one or more particles with a mass between 0.5 TeVand 1.5 TeV, the ILC would not be able to “see them”, whereasthe CLIC would. The cost of construction is comparable to thatof the ILC (although the project is less “mature” and so thefigures are less certain), but the operating costs wouldprobably be much higher.As far as circular accelerators are concerned, studies regardingthe Muon Collider initiated by the USA in 2014 have runaground. All energy is therefore now being concentrated on thenew FCC (Future Circular Collider) project proposed by CERN.Based on a 100 km underground ring to be constructed nearGeneva, it would use all CERN’s existing infrastructure, as wellas requiring a great deal more. The idea is to create an “all-inclusive” ring: the 100 TeV centre of mass hadron collider forproton-proton collisions (and heavy ions as in the LHC), FCC-hhcould also follow a 350 GeV (0.35 TeV) centre of masselectron-positron collider FCC-ee. In the FCC it would thereforealso be possible to obtain electron-proton and electron-ioncollisions (FCC-he). The lepton collider, whose energy is limitedby the enormous synchrotron radiation released by theelectrons, could produce the luminosity required by particlephysicists more rapidly and more surely than the ILC. The FCC-hh hadron collider, on the other hand, requires considerabletechnological investment, given that it is based onsuperconducting magnets similar to those that would beneeded for the HILUMI, but even more powerful.A value of 15-16 tesla, or even 20 tesla (although the latter is,for now, only a preliminary hypothesis) is required in order to

reduce the machine’s radius or increase its energy. This workwill entail an additional period of research and developmentbesides that necessary for HILUMI. The FCC could be approvedin around 2022-2025 and be ready to start in about 2040, although the lepton FCC-ee collider might be ready some yearsbefore then, since no specific technological developments arerequired. The challenge is formidable but there is in principleno reason why 15-16 tesla should not be attainable using theniobium-3-tin magnets of the HILUMI project. On the otherhand, the possibility of reaching and exceeding 20 tesladepends on the development of high-temperaturesuperconducting materials which are currently very costly andnot yet suitable for use in magnets for particle accelerators. Asregards the physics of particle accelerators, FCC is a more“traditional and secure” option, including in terms ofluminosity: everything that is learnt with HILUMI is immediatelytransferable to FCC-hh. The costs are not known, the study isonly just starting, but the figure could stand at around € 15billion. One advantage lies in the use of remarkable existinginfrastructure, CERN and the LHC, another is that major newinfrastructure (tunnels, cryogenics, etc) can be shared by thedifferent proposed accelerators. An intermediate step on theroad towards the FCC could be to double the energy of theLHC, a project proposed in 2010 under the name of HighEnergy LHC, using 16 and 20 tesla magnets to reach energiesof 26 and 33 TeV, to be installed in place of the LHC in thesame tunnel, thus reducing infrastructure costs to a minimum.Another stimulus to the challenge comes from China. A groupof Chinese physicists plan to build an accelerator with a 50-70km tunnel to the south of Shanghai. Their initial aim is toconstruct a circular lepton collider within a shorter time frame

c.Position of the 100 km FCC ringbetween Switzerland and France,in relation to the current LHC.



(from 2030), similar to FCC-ee (in strong competition with theILC that might be constructed in Japan). A subsequent phase(around 2040) envisages the construction of a “FCC-hh-like”accelerator, probably with energy of 60-80 TeV, in the tunnel. The critical time for a decision could be the renewal of theEuropean Strategy for High Energy Physics, scheduled for2018-2020. Meanwhile, new data collected by the experimentsin the LHC will have provided information that is currently notavailable and we will know more about the technologicalevolution of new high-field magnets and high-gradient radio-

frequency cavities. We will also have a better idea of the costsinvolved. The ILC could be the exception, as its future mainlyappears to be linked to a decision of the Japanesegovernment, which is expected within a year or two. SinceChina wants a super-accelerator and the machine that is “mostready” is the ILC, members of the scientific community arestarting to wonder whether a Chinese ILC might be best. In any case the “post-LHC” era is opening new scenarios thatwould have been unimaginable only a few years ago, trulyaccelerating our entry into the future.


d.The evolution of accelerators in time. Note the impactof superconductivity. The dates for future acceleratorsare clearly all highly hypothetical.

BiographyLucio Rossi has been a professor in Milan since 1992. Until 2001 he conducted research andprojects in the field of superconductivity applied to accelerators and detectors. In 2001 he joinedCERN, where he led the group involved in construction of the magnets for the LHC from 2001-07.He proposed the HILUMI project, for which he is project manager, and is among the proponents ofthe High Energy LHC and FCC projects.

Web links

http://tlep.web.cern.ch/

http://lpap.epfl.ch/page-54797-en.html

https://www.linearcollider.org/ILC


[as] reflexes

Magnets for the future.by Eleonora Cossi

ASG Superconductors is an Italian company that boasts ahistory of cutting edge technology in the field of theconventional and superconducting magnets that goes back tothe ’50s. The Magnets Unit of the Ansaldo industrial group,which became ASG Superconductors in 2001, has producedmagnets for the most important European laboratories andhigh-energy physics experiments, including the dipoles for theElettra project in Trieste and Hera in Hamburg, the quadrupolesfor the ESRF project in Grenoble and the bars and quadrupolesfor the LEP accelerator at CERN. During the construction of theLHC “technicians” at ASG made 400 magnets to be used inthe accelerator and detectors of the CMS and ATLASexperiments. Today the company is engaged in the productionof the new superconducting magnets for the HILUMI project.“Engineers at ASG have been working at CERN’s laboratoriesfor about a year now, designing the magnets that will allow theLHC to reach the highest luminosity values. Meanwhile,engineers at our workshop are also contributing by transferringthe construction methods necessary to develop the newaccelerator”, explained Vincenzo Giori, Managing Director ofASG Superconductors.

[as] Exactly which magnets is the company contributing to?

[Vincenzo Giori]: ASG is working on the design andconstruction of the quadrupole and dipole magnets. The former are responsible for the correct focusing of the beam during itsorbits in the accelerator, the latter for defining the actualtrajectory of the beam’s orbit.

[as] What characterises these new magnets?

[Vincenzo Giori]: The distinctive characteristic of the magnetswe’re working on is the type of superconducting material used,niobium-3-tin (Nb3Sn), which has never been used in acceleratormagnets before because it is a particularly complex componentto develop. Unlike niobium-titanium (Nb-Ti), which is used morefrequently, niobium-3-tin can withstand extremely high currentdensities, but has to be heat treated at 650°C, otherwise it is apoor conductor.

[as] There are promising applications for superconductivity infields ranging from thermo-nuclear fusion to the optimisationof electricity networks and medicine. Which applications doyou foresee for niobium-3-tin?

[Vincenzo Giori]: So far, niobium-3-tin is the only reliablematerial for producing magnetic fields of more than 20 tesla,which will have important implications for research into newmedicines. Nevertheless, we need to drastically reduce thecosts and make it much more practical to use if it is to be usefulin other sectors.


a.Vincenzo Giori, Managing Directorof ASG Superconductors based inGenoa.


Behind the scenesof the universeHypotheses about dark matter

by Gianfranco Bertone

If you look at the sky from a dark enoughplace, the Milky Way looks like a band ofdim light running across the firmament.The realisation that what James Joycecalled “infinite lattiginous scintillatinguncondensed milky way” is simply thedisk of stars and gas of the galaxy welive in, as seen “from the inside”, is arevelation that gives the sky a sense ofperspective, providing depth to theotherwise two-dimensional vault of theheavens.However, according to moderncosmology, in the galaxy we live in – theMilky Way – there is a lot more to seethan what is visible to the naked eye. Notonly is there more matter than even ourmost powerful telescopes would be ableto observe, but, as we now know, thatmatter is not made of stars, planets,

comets, asteroids or gas: it is somethingfundamentally different from any othersubstance that has ever been observedin our laboratories. It is called darkmatter, because it neither emits norabsorbs light, but the name is especiallyappropriate for a substance whosenature is wrapped in mystery. It took the scientific community a longtime to come to this conclusion, buttowards the end of the ’70s, afterdecades of pioneering research, proof ofthe existence of dark matter waspractically irrefutable. Added to themeasurements that were available then,such as anomalies in the speed ofrotation of stars and gas at hugedistances from the galactic centres andthe inexplicable speed of galaxiesgrouped together in big masses, we now

also have what is called backgroundcosmic radiation, discovered in 1964 byArno Penzias and Robert Wilson andmeasured with extraordinary precision bysatellites like NASA’s WMAP in 2003,and recently by Planck, launched by theEuropean Space Agency.Just like an iceberg of which only the tipis visible, we know, thanks to the laws ofphysics, that the part above the surface,the universe with which we are familiar,represents only a small part, around 5%of all matter-energy. Identifying thenature of the remaining 22%, made up ofdark matter, and of the 73% made up ofdark energy (see p. 31, ed.), is one ofthe chief objectives of moderncosmology.But how do you look for something youknow nothing about? One of the mostinteresting ideas, and perhaps the oneon which the scientific community iscurrently concentrating most of itsattention, is that dark matter iscomposed of a new type of elementaryparticles, generically called WIMPs(Weakly Interacting Massive Particles).WIMPs are interesting for severalreasons: their existence is predicted bynew theories of particle physics – suchas supersymmetry – which try to explainand extend the standard model ofparticle physics; they might easily have


a.The galaxy we live in, the Milky Way,is made up of a disk of stars andgas, which can be seen with thenaked eye and looks like a ribbon oflight in the sky, and a huge quantityof invisible matter, dark matter,which supports its structure.


been produced in the right quantity a fewmoments after the Big Bang; and couldbe discovered through a series ofexperiments that are ongoing or will beconducted in the near future.Of these experiments, two of the biggestand potentially most interesting areATLAS and CMS at the LHC, whichdiscovered the Higgs boson in 2012. Ifdark matter is composed of WIMPs, itmight be produced in collisions betweenprotons accelerated to the highestenergies and made to collide at thecentre of the ATLAS and CMSexperiments. All known particles leave atrace in the sensors of these twoexperiments, but not WIMPs, whichdisappear without leaving any trace,taking away part of the energy of thecollision and thus manifesting theirpresence (see p. 16, ed.).A second group of experiments, knownas “direct detection”, look for the energydeposited by the dark matter particlesthat penetrate them and collide with theatomic nuclei inside them. However,there are two big problems with directdetection. The first is that the darkmatter particles interact weakly and sothe experiments have to be extremelysensitive and big enough to detect astatistically relevant number of events.The second is that very many “ordinary”particles, originating from deep space,penetrate the detector and so continuallyactivate it. The experiments must betaken to underground laboratories, suchas the Gran Sasso National Laboratory(in Italy), to shield these cosmic rays.Tens of experiments are currentlyunderway. One of these is the DAMAexperiment, which has observed for thefirst time an annual modulation in thenumber of events detected, a

characteristic scientists expect to beconsistent with the presence of darkmatter, since during the Earth’s annualrevolution around the Sun, a “wind” ofdark matter particles blows and varies instrength according to the speed of theEarth with respect to the galaxy. A newgeneration experiment called Xenon1Tonwill start this year and become the mostsensitive in the world. Lastly, dark mattercan be identified “indirectly”, by meansof the particles – for example antimatterparticles – produced by specificinteractions called self-annihilation, inwhich two particles of dark mattercollide, transforming their mass intoenergy. This approach has producedsome interesting results, including thediscovery of an excess of gamma rays,i.e., very high energy photons, observedby the Fermi space telescope towardsthe centre of the Galaxy, withcharacteristics that appear to signal thepresence of dark matter, and the recent

findings of AMS-02 (see p. 27, ed.).In addition to WIMPs, many othersolutions have been proposed to explaindark matter. Scientific articles are full ofhypothetical particles such as axions,sterile neutrinos, mirror particles andmany others with even more exoticnames (see fig. b), a bit like themythological and imaginary creatures ofmedieval bestiaries. Just as some ofthose "monsters" turned out to beversions, albeit distorted, of realanimals, so modern science is searchingamong the particles imagined bytheoretical physicists from all over theworld for the ones that would explain themystery of dark matter in the universe.And while we travel further on thisjourney suspended between the infinitelysmall and the infinitely big, every day welearn something more about thestructure and evolution of the universeand our role in it.

b.Theoretical physicists haveproposed different possiblecandidates for the role of darkmatter. Some of these are shownin the diagram, with the possiblevalues of their mass plotted on thehorizontal axis and the probabilityof interaction with ordinary matter,measured by the collision cross-section, plotted on the verticalaxis. WIMPs are the most activelysought candidates.


BiographyGianfranco Bertone is a professor at the University of Amsterdam, where he coordinates theactivities of the Gravitation Astroparticle Physics Centre of Excellence. He is author of the bookBehind the Scenes of the Universe and of the related Dark Matter app available on iTunes.

Web links

http://www.lngs.infn.it/en/dark-matter

http://www.perimeterinstitute.ca/videos/behind-scenes-universe-higgs-dark-matter


News from the spacestationAntimatter and dark matter being studied by AMS

by Bruna Bertucci

“[…] we must regard it rather as an accident that the Earth (and presumably the whole solarsystem) contains a preponderance of negative electrons and positive protons. It is quitepossible that for some of the stars it is the other way about, these stars being built up mainlyof positrons (positive electrons, ed.) and negative protons...” This is how Paul Dirac ended hisspeech when he received the Nobel Prize on 12 December 1933.The conclusion of Dirac’s speech ideally represents the starting point for the Alpha MagneticSpectrometer (AMS-02) experiment, in which the INFN (Italian National Institute for NuclearPhysics) and ASI (Italian Space Agency) are taking part. The instrument, located on the ISS(International Space Station), is designed to pick up the weak signals of antimatter particlesin the continual flux of cosmic rays passing through the space around the Earth.Detecting these signals that belong to the infinitely small can supply the necessary clues tosolve some of the unsolved mysteries about the infinitely big: the universe.The first mystery concerns the predominance of “matter” particles around us (baryogenesis):where did the antimatter particles go? We have been led to believe that in the immediateaftermath of the Big Bang the universe was “symmetric”, populated in equal measure by theelementary particles of matter and antimatter. Their natural and violent annihilation apparently

27

a.A selfie of an astronaut taken inorbit: at the top on the left the AMS-02 experiment is visible on the ISSstructure.



resulted in the disappearance of theentire population of antiparticles and theuniverse therefore evolved starting fromthe particles of matter – one in a billion –that survived the annihilation. Notheoretical justifications have so far beenable to adequately explain the origin ofthis asymmetry. So finding antiheliumnuclei, or the antinuclei of heavierelements, that cannot be producednaturally in a universe built of matteralone, would open up a new scenario inwhich antimatter had not disappeared,but was simply confined to regions of theuniverse distant from Earth.The second mystery deals with the natureof the matter that makes up our universe.Only 5% of the mass-energy content ofour universe is attributable to theordinary matter we are made of, basicallyprotons, neutrons, and electrons, whilearound 22% is made up of dark matterparticles, new types of elementaryparticles that interact weakly withordinary matter and are therefore“invisible” to telescopes sensitive to thelight produced by the electromagneticinteractions of matter. But rare collisions

of dark matter particles (that annihilatewhen they collide) can generate photons,particles and antiparticles of ordinarymatter (electrons/positrons,protons/antiprotons), whose flux overlapsthat of the cosmic rays. Although thereare different theories and hypothesesabout the nature of dark matter and thereis a great deal of uncertainty about theexpected number of collisions, in all thepossible scenarios the fluxes of particlesproduced by the collisions of dark matterare several orders of size smaller thanthose of cosmic ray particles, mainlyprotons, helium nuclei and heavierelements with a small percentage (1%) ofelectrons. Fluxes of antiparticles, whichare particularly weak in ordinary cosmicrays, offer the only hope of finding anysign of dark matter. Collisions of cosmicrays with the interstellar medium areexpected to produce about one antiprotonfor every 10,000 protons and onepositron for every 10 electrons: thesefluxes are comparable with those of darkmatter and thus constitute a “base” fromwhich to fight on equal terms in the huntfor new phenomena.

In terms of experiments, the firstchallenge in searching for weak signalslike antimatter particles is to interceptthe cosmic rays before they have thepossibility of interacting with the Earth’satmosphere and generating a flux ofantimatter particles that could falsify themeasurement. That is why detectorsused in this type of research are sent tothe upper layers of the atmosphere usingaerostatic balloons or sent into orbit onspace satellites, such as Pamela orFermi, or onboard the ISS, like AMS-02.The second challenge is to gather asignificant sample of particles. Theexpected number of antiparticles at therelevant energies in order to identify newphenomena is a few hundred events ayear for exposed surfaces of about onesquare metre. However, owing to weightlimits and the electric power needed tooperate equipment in space, the size ofthe equipment cannot be increased atwill. Therefore, the only alternative is todesign equipment capable of ensuringyears of highly efficient operation.But the fundamental point is to succeedin identifying the rare antiparticles with


b.The AMS-02 experiment at NASA’sKennedy Space Center shortlybefore being placed inside theEndeavour’s payload bay.


precision: electrons and protons only differ from theirantiparticles for the sign of the electric charge. The only way toseparate particles and antiparticles is to use a magnetic fieldable to bend positive and negative particles in oppositedirections. Positrons and protons both have the same positiveelectric charge value but there is only one positron for every1000 protons. To separate these, and to separate theantiprotons from the more abundant electrons, we need toexploit the different ways in which the two types interact withmatter.The AMS-02 experiment was therefore designed as a magneticspectrometer. At its heart it has a permanent magnet and atrace detector made up of around 6.4m2 of silicon micro-stripsensors arranged in nine different layers, capable of measuringthe position of the particles that pass through the apparatuswith an accuracy of 10 micrometres. The apparatus includesfive other instruments that perform complementary andindependent measurements to identify the particles passingthrough them with precision.The techniques used and the complexity of the apparatus arecomparable to those of the latest instruments used in particleaccelerators, but have been adapted to work in a spaceenvironment. The AMS-02 has been operating continuously sinceMay 2011 on-board the ISS, in orbit at a distance of some 400km from the Earth, absorbing a total power of 2kW - less thana washing machine! Measuring 3 metres in width, 3 metres inlength and 5 metres in height, and with an overall weight ofaround 7 tonnes, it is a giant in space, but is a small fractionof the experiments in the LHC.

The experiment is controlled at a distance, through the NASAsatellite network, which allows scientists to communicate withthe ISS and download data from its 300,000 channels ofelectronics. Physicists in the CERN control room and at itstwin facility in Taiwan work in shifts to monitor the apparatus24/7 and act in real time, adjusting the experiment’s controlparameters as necessary according to conditions on-board thestation.In the first 30 months of data taking, AMS-02 has collectedsignals from around 40 billion cosmic particles, more than allthe experiments conducted in the course of the century sincecosmic rays were first discovered. Among these particles,about 10 million electrons and around one million positronshave been identified, and used to measure the ratio of thenumber of positrons to the combined number of electrons andpositrons, reaching a previously unexplored energy limit forthese components of cosmic rays. This result confirms anexcess of positrons with respect to the expected naturalabundance in cosmic rays, as already observed by Pamela andin the first 18 months of operation of AMS-02, by extending themeasured energy range and improving the degree of precision.This is of extreme importance in order to create an identikit ofpossible sources of antimatter. One of these sources mightwell be collisions of dark matter (see “in depth”).But is the increase in the fraction of positrons due to anadditional source of positrons or to the “disappearance” ofelectrons? The observation of the electron and positron fluxes separatelycharacterises its trend as a function of energy with extreme

29

c.The space shuttle Endeavour, whichcarried AMS-02 to the ISS in 2011,on the launching pad at KennedySpace Center in Cape Canaveral,Florida.



precision. The results clearly indicate that there are no abruptspectral variations in the electron flux, thus confirming that thebehaviour of the positron component as a function of energyrequires the presence of new phenomena for their production.These observations are consistent with the flux of positronsgenerated in collisions of dark matter particles (especially,neutralinos) with a mass on the order of 1 TeV. To determinewhether the positron excess really does come from dark matteror from astrophysical sources, for example pulsars close to ourplanet, we need to measure the rate of decrease at which thepositron fraction falls (see fig. 1) and then compare the effectthat is observed with that measured in other components ofantimatter, such as antiprotons.

On the other hand, the mission of AMS-02 has only just begunand the experiment will continue to collect data for the entireoperational life of the ISS. It will carry on its observations andmeasurements for another decade or so, continuing to searchfor antimatter and observe or definitively exclude the presenceof antihelium in our universe. At the same time,measurements to determine the composition and energyspectrum of ordinary cosmic rays will enable us to makeprogress in the study of their sources and the mechanismswhereby they pass through the galaxy, heliosphere and Earth’smagnetosphere to reach us.


BiographyBruna Bertucci is professor of physics at the University of Perugia. Her research initially addressed the fieldof elementary particles when she took part in the LEP experiments at CERN in the late ’80s. Since the late’90s she has mainly focused on the experimental study of cosmic rays in space, first with the AMS-01experiment and then with AMS-02, of which she is currently the Italian project manager..

Web links

www.ams02.org

http://www.asdc.asi.it/

[as] in depth

Too many positrons

As shown in fig. 1, the positron fraction observed by AMS-02 increases rapidlystarting from an energy of 8 GeV, indicating the existence of a new source ofthis component of antimatter, with respect to the expected “standard”production of positrons in cosmic rays. The excess of the positron fraction isisotropic (that is, it has the same intensity regardless of the direction ofmeasurement) within 3%, suggesting that this excess might not originate fromspecific directions.

A detailed analysis of the positron fraction as a function of energy shows thisto increase gradually, excluding sudden variations, peaks or valleys, and theenergy at which it ceases to increase has been measured to be around 275GeV. Observations will continue over the coming years and be extended to evenhigher energies. This should improve our understanding of the nature of thephenomenon being observed and help us to describe its characteristics evenmore accurately.

1.Graph of the measurement performed by AMS-02 of the positron fraction in relation tothe sum of electrons and positrons (red dots). The expected trend in cosmic rays isshown in blue. The green line best describes the experimental data.


… that moves the sunand other stars Cosmological constant, dark energy and expansionof the universe

by Luca Amendola

In the first pages of De Caelo, Aristotle,like many people before him, asks a verynatural question: why don’t the sky andthe stars fall down on us? But because hewas Aristotle and not just any distractednight owl, he immediately goes on to ask:why don’t the stars move away from us?The idea of the universe as an evolvingsystem thus began to emerge in theminds of great thinkers. But as Bohronce said, the opposite of a grand idea isanother grand idea. For the next two

millennia philosophers, theologists andthe first modern scientists were almostunanimous in affirming that the cosmoswas in actual fact a static entity with aneternal and motionless structure.Einstein himself, in his earlier works oncosmology, did no better than Aristotle,almost repeating his same words: the skyis unchangeable, therefore the universe isstatic. But, being Einstein and no meresophist, he soon understood that the onlyway to keep the universe static was tofind a way of resisting the universal forceof gravity. Since gravity is alwaysattractive and acts on every particle, the concept of static distribution wouldmean that matter would collapse onitself. The solution conceived by Einsteinin 1917, the famous cosmologicalconstant, marked the start of a brilliant,multi-faceted career. A few years later, anumber of astronomers, led by EdwinHubble, discovered that the universe isexpanding or, more correctly, that all thegalaxies are moving and that motion ismaking them move further apart. Thisofficially marked the beginning of moderncosmology, based on data rather than onspeculations.There was no need for the cosmologicalconstant to explain the non-acceleratedexpansion that appeared to be observedat the time, and Einstein’s idea remained dormant for several decades.Dormant but not forgotten because, evenif not required, the cosmological constantis like the genie of the lamp: once out, itdoesn't want to go back in. According to quantum physics of fieldsthe cosmological constant is an intrinsic

a.The geocentric (or Ptolemaic)system described by Aristotle in DeCaelo, a vision almost universallyacknowledged for about twomillennia by scholars who believedthe cosmos was a static entitywhose structure was eternal andmotionless.



property of a vacuum and there is noevident reason why this should be null.On the contrary: calculations that can bedone on a paper napkin show that this“vacuum energy” should be huge, hugeenough to immediately make the entireuniverse “explode” or “collapse”. Thoughsomewhat absurd, such calculations dopoint to the fact that there is somethingdeep down in the cosmological constantthat we are completely missing.The most spectacular come-back in thehistory of physics happenedunexpectedly in 1998. Two groups ofastronomers and physicists, led by SaulPerlmutter, Brian Schmidt and AdamRiess, published the results of a decade-long study. Just like Hubble fifty yearsearlier, the two groups had measured thevelocity and distance of objects that werevery far away, not galaxies this time, buttype Ia supernovae, observable atdistances a thousand times greater thanHubble supernovae. These supernovaeexplode when they reach a fixedthreshold, called the Chandrasekharmass. Their maximum luminosity istherefore relatively constant, regardlessof the details of the explosion. Bymeasuring the amount of light thatreaches our telescopes, we can directlyestimate the distance of the sources,because the amount of light collected

depends inversely on the supernova’sdistance squared, with the appropriatecorrection needed for cosmic expansion.The conclusions reached by the tworesearch groups shocked scientists: theirdata clearly showed a universe that wasaccelerating, inexplicable without acosmological constant or something verysimilar. This meant new cosmicexpansion, not something confined to thedistant past, but taking place before ourvery eyes.In actual fact, a whole host of diabolicaldetails were undeservedly condensedinto that adverb, “clearly”. Before thecosmological constant could bedemonstrated (and the Nobel Prize couldbe awarded to Perlmutter, Schmidt andReiss, in 2011), the results had to bechecked and all possible alternativeexplanations examined.The cosmological constant, universallyindicated with the symbol Λ, has manymysterious properties. The mostimportant is that it is a form of energythat does not fade with distance, butremains constant. This is a consequenceof another unusual characteristic, itsstrong negative pressure. According tothe theory of general relativity, pressureexerts gravitational force just like mass.Because the resulting force is negative,it speeds the expansion up instead of

slowing it down like ordinary matter anddark matter do. It therefore acts as asort of antigravity, which is not veryconvenient for science fiction writers,because since it is absolutelyhomogeneous, it cannot be moulded atwill to make antiplanets, antistars andantigravitation motors.One immediate consequence of its valuebeing independent of time is that thecosmological constant will continue toaccelerate this expansion forever, evenwhen the density of matter and radiationhave fallen to imperceptible levels. Todayit is estimated that the Λ constant isresponsible for 73% of the universe’senergy and that this percentage is set torise over the next billions of years owingto the expansion of the universe.The results of the supernovae were soonconfirmed by many other observations,from those measuring cosmic microwavebackground radiation to deviations in thedistribution of galaxies. Nonetheless,despite increasingly high levels ofprecision, the data obtained so far arenot sufficient to establish once and forall whether or not the Λ constant is theonly possible explanation.To bridge the gap caused by thisuncertainty, and in view of thefundamental problems that quantumphysics associates with vacuum energy,

b.The evolution of cosmic distancessince the Big Bang. After the initialexpansionary phase (not shownhere) and subsequent slowing, theexpansion of space started toaccelerate due to the effect of darkenergy. Of the various hypothesesabout future trends, thoseillustrated here are the Big Crunch,where everything collapses backonto itself, and the Big Rip, wherethe universe expands so quicklythat all physical structures, fromstars to atoms, are ripped apart.



cosmologists have produced many otherinteresting hypotheses.Just as the primordial expansion of theuniverse may have been triggered by a“particle” or rather a field, called inflaton,so its recent acceleration might be dueinstead to the cosmological constant, thehidden activity of a field/particle calleddark energy or quintessence (back comesAristotle!) or simply scalar field.Like all fields, this extends andpropagates throughout space and has itsown dynamics. Like all particles, darkenergy also has a mass, albeit so smallthat no accelerator can measure itdirectly: its wavelength is such that this

particle is truly impalpable, distributedover distances equal to the entireuniverse.Dark energy or quintessence resemblesthe cosmological constant, but it is notexactly constant and its density thereforevaries slowly in time and can evenfluctuate and increase slightly in space.Lastly, there is another appealingpossibility: the accelerating expansion ofthe universe might actually be due to anew dark force, capable of acting directlyon matter, in the same way as gravity,electromagnetic fields or twofundamental nuclear forces. This “fifthforce” is almost indistinguishable from

gravity itself, so much so that it is evencalled modified gravity. A modified gravitycould have innumerable consequences:the whole epic of the universe wouldhave to be rewritten to account for apowerful new factor, going well beyondthe mere cosmological constant.The observational and theoretical effortsof cosmologists all centre around thesehypotheses. One of the most ambitiousprojects is the ESA Euclid satellitemission, to which Italy is making animportant contribution, scheduled to belaunched in 2020. Euclid will be acosmology telescope that, over fiveyears, will catalogue the spectrums (andtherefore the distance using redshiftinformation) of 50 million galaxies andimages of a further two billion, in orderto create an accurate three dimensionalcosmic map in a volume equal to a cubewith each side measuring one billion lightyears. The galaxy distribution will becompared against those predicted by thevarious theories on dark energy andcombined with all the data as theygradually become available. The resultwill be a high-precision measurement ofall the main cosmological parameters,including the density of matter and darkenergy, the mass of neutrinos, theprimordial energy density fluctuationspectrum and the rate of expansion atdifferent distances.The hope is that we will at least be ableto read the identity of the cosmologicalconstant or dark energy or modifiedgravity in the folds of Euclid’s map. Thecertainty is that Euclid and other similarexperiments will provide us with a broaderand deeper vision of our universe andhelp us to at last understand why the skydoesn’t fall on our heads.

BiographyLuca Amendola has been an astronomy researcher at theItalian Institute for Astrophysics (INAF, Rome Observatory)until 2009. He is currently professor of theoretical physicsat the University of Heidelberg (Germany). He has spentperiods doing research at FermiLab in Chicago, in France,Germany and Japan. He is the author of an educationalbook [Il Cielo Infinito (The Infinite Sky), Sperling].

Web links

http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

http://hubblesite.org/hubble_discoveries/dark_energy/de-what_is_dark_energy.php

http://sci.esa.int/euclid/42267-science/

c.Artistic image of the Euclid satellite.



Elusive mysteriesMass and nature of neutrinos

by Carlo Giunti

Neutrinos are the most mysterious elementary particlesknown to us. They are among the fundamental particlesof the standard model and their formulation hastraditionally been based on their characteristics ofinteraction. However, 85 years after Wolfgang Pauliformulated his hypothesis and 59 years after the firstexperimental observation by Clyde L. Cowan andFrederick Reines (see “in depth”), we still do not knowsome of their fundamental properties: the value of theirmass, their nature (whether they are of so-called Dirac orMajorana type) or their number (i.e., whether there arenon-interacting, or sterile, neutrinos in addition to thethree "active" neutrinos, the electron, muon and tauneutrino, known to interact with mattaer). There are alsogood reasons to believe that the unknowncharacteristics of neutrinos are linked to new physicsbeyond the standard model.One quantity of fundamental importance for everyparticle, like for every body, is its mass, whichdetermines its propagation and interaction properties.At present we know the value of the mass of all theparticles in the standard model except neutrinos, themass of which is so minute that until about 15 yearsago we had no irrefutable proof that neutrinos were notmassless. This proof came from the observation ofneutrino oscillations, the mixing of neutrinos ofdifferent types (or flavours) which leads, for example,to the transformation of an electron neutrino into amuon neutrino. The phenomenon, proposed separately

a.Clyde L. Cowan and Frederick Reines(left) with the detector they used toobserve a neutrino for the first time atHanford (Washington) in 1956.



in the 1960s by Pontecorvo and Ziro Maki, Masami Nakagawaand Shoichi Sakata, depends on the distance the neutrinotravels, its energy and the difference in mass between theneutrino types.The measurement of these oscillations has enabled scientiststo calculate the differences between the masses of thedifferent neutrinos. These masses are minute; we know theyhave a mass, but it has not yet been possible to measuretheir absolute value. The experimental upper limit is around250 thousand times smaller than the mass of an electron,which is the lightest particle of matter in the standard model

apart from neutrinos. There must be an explanation for whyneutrinos have a far smaller mass than the other particles inthe standard model, but this cannot be derived in a naturalway within the standard model, because it would mean settingan artificially low limit for the parameters of the standardmodel that determine the masses of the neutrinos. Thesmallness of the mass of neutrinos is, instead, thought to bedue to their connection with the new physics, through theirproperty of being either Majorana particles (i.e., particleswhich coincide with their own corresponding antiparticle), orDirac particles like quarks and charged leptons (electrons,


[as] in depth

Ghost stories

In 1930 Pauli proposed the existence of neutrinos to explain the fact that innuclear decay due to the weak interaction (which is much slower than decay dueto strong and electromagnetic interactions) electrons are emitted with acontinuous spectrum of energy. This is only possible if there are at least three endproducts of the decay: the final nucleus, the electron and a neutrino, which wasfor a long time impossible to observe, because it is electrically neutral and onlyinteracts weakly with matter (whereas charged particles like electrons leavetraces in particle detectors due to ionisation of the atoms). Based on Pauli’shypothesis, in 1934 Enrico Fermi proposed the theory of weak interactionsaccording to which, however, neutrinos interact so weakly that it would be verydifficult, if not impossible, to directly verify their existence. Fortunately, thispessimistic hypothesis was proved wrong by the fact that some sources producehuge fluxes of neutrinos: for example, a nuclear reactor typically produces around1020 neutrinos a second per gigawatt of thermal energy and the Sun generatesapproximately 10 11 neutrinos a second per square centimetre (about thesurface of a fingernail). Therefore, even if the majority of neutrinos pass through adetector as if it were transparent, given the enormous flux of neutrinos it ispossible to observe some interactions that demonstrate the existence of theseparticles. This measurement was performed for the first time by Cowan and

Reines in 1956 using a detector installed close to a nuclear reactor.Their measurement finally proved the existence of an electron neutrino emittedtogether with an electron in the nuclear decay that occurs in a reactor. Meanwhilehowever, in 1937, the muon was discovered: a charged particle similar to anelectron, but around 200 times heavier. In 1960 Bruno Pontecorvo proposed theexistence of a second type of neutrino produced with a muon. This hypothesiswas brilliantly confirmed in 1962 by the experiment conducted by LeonLederman, Melvin Schwartz and Jack Steinberger, who demonstrated thatneutrinos produced by weak interactions together with muons do not produceelectrons when they interact with matter, as would electron neutrinos. They aretherefore different particles, called muon neutrinos. Later, in 1975, a thirdcharged lepton called the tau was discovered. The tau is the heavier brother ofthe electron and muon (around 17 times heavier than a muon) and thecorresponding neutrino of the tau was observed in 2000. This completes the listof the three known neutrinos, which are active in the weak interaction processesthat led to their discovery. There is still a possibility of finding other types ofneutrinos, called sterile neutrinos, which are not associated with any chargedparticles in the standard model.

1.Bruno Pontecorvo (right), one of the famous “ViaPanisperna boys”, in conversation with his collaboratorSamoil Bilenky. Pontecorvo hypothesised the existenceof a second type of neutrino in 1960.


muons and tau). Let us try to understandwhat that means.In 1928 Paul Dirac proposed hisrelativistic quantum theory of fermions,like the electron, which for this reason arecalled Dirac particles. A fundamentalcharacteristic of a Dirac particle is thatthe particle state (for example, theelectron, which has a negative electriccharge) always has a correspondingantiparticle state with the oppositeelectric charge (the positron in the case ofthe electron, which has a positive electriccharge). Quarks and charged leptons(electrons, muons and taus) are Diracparticles, whereas neutrinos (that areneutral) may be Majorana particles,according to the theory developed in 1937by Ettore Majorana. With Majoranaparticles, the state of the particle is thesame as that of the antiparticle. This isonly possible for neutral particles likeneutrinos, while for charged particles theparticle and antiparticle states arenecessarily distinct since they haveopposite electric charges.Within the framework of the standardmodel, massive neutrinos can only be Dirac particles, because the Higgsmechanism that gives mass to particlescan only do so for Dirac particles. For thisreason it is extremely interesting toexperimentally determine whether massiveneutrinos are Majorana particles, becausein this case the masses of neutrinos mustbe generated by a mechanism of new

physics. Furthermore, if neutrinos areMajorana particles, the see-sawmechanism can be used to explain thesmallness of their masses. Thismechanism is based on the existence ofnew physics beyond the standard modelat a very high energy scale, for example,the scale of the Grand Unification Theory(GUT) of the strong and electroweakforces, that is to say, on the order of 1015

-1016 GeV, much higher than theelectroweak energy scale, which is on theorder of 100 GeV, at which the standardmodel unification of the electromagneticand weak forces takes place. According tothe see-saw mechanism (see fig. b) themasses of neutrinos are proportional tothe relationship between the square of theelectroweak energy scale and the GUTenergy scale, with a value ofapproximately one hundredth of an eV (10-

2 eV), which is precisely the expectedvalue of the masses of neutrinos.Thus, if neutrinos are Majorana particles,their masses establish a link between thephysics of the standard model and thenew physics.The experiments most sensitive to thesmall masses of Majorana neutrinos arethose which try to measure an extremelyrare process called neutrinoless doublebeta decay involving certain heavy nuclei,such as isotopes of germanium andtellurium, used in the GERDA and CUOREexperiments at the INFN’s Gran SassoNational Laboratory.

b.Illustration of the see-sawmechanism: the greater (heavier)the energy scale of the physicsbeyond the standard model (100GeV), represented by the elephant(for example, the energy scale ofthe Grand Unification equal to 1015

GeV) the smaller (lighter) themasses of Majorana neutrinos(represented by the mouse).According to the energy scale of thegrand unification, the mass of aMajorana neutrino would be equalto 10-11 GeV.



New physics might even manifest itselfthrough new, very light particles that,being neutral and not interacting with theweak force of the standard model, wouldappear to us as sterile neutrinos (a namecoined by Pontecorvo in 1967). In thiscase the three active neutrinos, which“respond” to the weak interactions,through which they are produced anddetected by physicists, can oscillate intosterile neutrinos, which eludeexperimental detection. This phenomenonmight explain recent reports of failure todetect the measured flux of neutrinosgenerated in nuclear reactors andradioactive sources. The SOX experiment,using the BOREXINO detector at the GranSasso facility, will verify the validity ofthese findings in the next few years using

radioactive neutrino sources. Thismeasurement is clearly of the utmostimportance in order to study the physicsbeyond the standard model, because apositive result would provide directinformation about the existence of a newparticle, the sterile neutrino, which doesnot belong to the standard model andwhose minute mass must be generated bya new physics mechanism.Research investigating the properties ofneutrinos, which are unique among theparticles of the standard model, providesa window onto the new physics that isdifficult to open because neutrinos are soelusive. Nonetheless, given theresourcefulness and tenacity of physicists,we can afford to be optimistic that this willhappen in the near future.

c.A researcher at work on the CUOREexperiment at the INFN’s GranSasso Laboratory.


BiographyCarlo Giunti is a researcher at the Turin division of the INFN. He is mainlyconcerned with research into the physics of neutrinos, which is the subject of hisbook, Fundamentals of Neutrino Physics and Astrophysics (Oxford UniversityPress, 2007).

Web links

http://www.nu.to.infn.it/

http://www.hep.anl.gov/ndk/hypertext/

http://pcbat1.mi.infn.it/~battist/cgi-bin/oscil/index.r


On the rocksNeutrino telescopes under the ice

by Elisa Bernardini

Neutrinos are among the most elusiverepresentatives of the world ofelementary physics. With no electriccharge and a minute mass, they are ableto penetrate dense layers of matterwithout leaving any trace. Unaffected bythe magnetic fields that permeate ourgalaxy and intergalactic space, theytravel towards Earth following lineartrajectories through which we are linkedto their sources. Neutrinos mightrepresent the only messengers forstudying the spectacular events thatproduce extremely high-energy particlesin the universe, which escape

observation by traditional astronomybased only on the detection ofelectromagnetic waves. Would theirdiscovery help us to solve puzzlingquestions like: how do stars explode?What happens in the proximity of a blackhole? Where do the cosmic rays thatreach Earth come from?Neutrinos do, albeit rarely, interact withmatter. And it is thanks to such collisionsthat we are able to observe them. Theprobability of neutrinos at energies ofseveral million eVs interacting when theypass through Earth is in the order of onein a hundred billion. Even at higher

energies, experiments to detect rareneutrino signals would take up adisproportionate amount of space, evenmore than the massive particle detectorsat CERN in Geneva. Many neutrinosreach the Earth every second. Most ofthem, at lower energies of between a fewthousand and a few million eV, comefrom the Sun and from stellarexplosions, so called supernovae. Atenergies of up to about one PeV (1015

eV) neutrinos are produced in theinteractions of the cosmic rays with thenuclei of the Earth’s atmosphere and arecalled atmospheric. Collisions inside

a.Artistic illustration of the ActiveGalactic Nucleus (AGN) of the ARP220 Galaxy. Near the black holeand in the streams, the cosmic rayscan be accelerated and neutrinoscan be produced at extremely highenergies.



astrophysical sources between accelerated protons and low-energy protons or photons can also produce high-energyneutrinos, although their numbers are several orders ofmagnitude smaller than in the previous cases. These areknown as cosmic or astrophysical neutrinos.IceCube, the biggest particle detector in the world, isdistributed over one cubic kilometre of ice to detect neutrinos

coming from the hidden depths of space. Researchers frommore than 40 research centres in 12 countries have lowered86 steel strings into the Antarctic ice sheet. Attached to thestrings are 5160 optical sensors, a network of “electroniceyes” ready to capture the passage of a neutrino. At a depth ofbetween 1450 and 2450 metres, in absolute darkness andsilence, the weak light impulses induced by rare collisions


[as] in depth

Detectors in the abysses

The idea of using huge natural volumes such as water or ice dates back to the’60s. The rapid flashes of blue light, called Cherenkov light, emitted by thecharged particles produced by the collision of a neutrino with a nucleus ofmatter, can be intercepted at great distances in a transparent medium. Byrecording the time of arrival and intensity of this radiation, scientists can tracethe direction of the source and the energy of the neutrino. The first attempt tobuild an underwater optical detector was made in Hawaii in the ’70s, with theDUMAND experiment. Although it was cancelled, this was an importantpioneering project and was tried again in the ’80s, this time more successfully,in lake Baikal in Siberia (Russia) (see fig. 1). The thick layer of ice facilitatedinstallation and maintenance operations during the winter season, and 200optical sensors were installed on 8 separate lines. Baikal was the firstexperiment to detect atmospheric neutrinos underwater. It is still running andwork is now underway to create a kilometre-scale observatory (called the

Gigaton Volume Detector). The open sea is also a transparent medium that isideal for detecting neutrinos and the KM3NeT project, which recently deployedthe first underwater structures off the coast of Capo Passero in Sicily, is lookingto develop a neutrino telescope three times bigger than IceCube in the NorthernHemisphere.Projects in the Antarctic ice have been no less pioneering. AMANDA, conductedtowards the end of the ’80s, was the first experiment in the South Pole. Thoughbased on the same kind of technology as the DUMAND experiment, differentmethods had to be devised to house it in the ice. The first sensors were installedat a maximum depth of one kilometre, where the Antarctic ice is still permeatedwith air bubbles that affect its transparency. The sensors therefore had to beinstalled at greater depths and it was only towards the end of the ’90s thatscientists were able to demonstrate their operation under the ice. The baton waspassed to IceCube in 2005.

1.The strings for the Baikal experiment are laid in the middle of winter, whenthe surface of the lake is frozen. Holes about 2 metres across are cut in theice layer and the strings are installed with the help of pulleys. The differentparts of the apparatus are connected to one another by watertightelectrical and optical cables, capable of withstanding the pressure exertedby around 1500 metres of water that sits on top of the apparatus.


between neutrinos are observed. IceCube identifies oneatmospheric neutrino about every six minutes, but its mainpurpose is to identify astrophysical neutrinos produced by thehuge astrophysical mechanisms that accelerate cosmic rays.The main “suspects” are the most violent phenomena in theuniverse, like active galactic nuclei (AGN) (see fig. a., ed.),galaxies with enormous radiation power presumably fed by theblack holes at their centre, and gamma ray bursts (GRBs),frightening flashes of radiation that last from under a second toseveral minutes, so energetic that they can reach us from theconfines of the observable universe. The validity of theseconjectures can be tested by observing the high-energy gammarays emitted by these sources. But even just observing theneutrinos would demonstrate that these sources accelerate theprotons and ions observed on Earth to extreme energies.The search for cosmic messengers was crowned with successin 2012, seven years after construction started, when morethan 260 international researchers involved in the IceCubeexperiment discovered two extraterrestrial neutrinos, whichthey named Ernie and Bert, from the famous television seriesThe Muppet Show. The two neutrinos had an unusually highenergy, more than one PeV. This aspect distinguished themfrom atmospheric neutrinos. After the detection of the lower-energy neutrinos from Supernova SN1987A in 1987, this wasthe second time that neutrinos coming from outside the solarsystem had been observed. The following year, in a fascinatingrace for success, a second careful analysis of data collected

over two years by IceCube provided evidence of another 26events, at slightly lower energies starting from around one TeV.A third year of data taking was added shortly afterwards,revealing a total of 37 events, including the neutrino named BigBird after the popular American TV series which beat allprevious records with its two PeV of energy! It is not only their higher energy, but also their angulardistribution that distinguishes these events from neutrinoscoming from the Earth’s atmosphere. The production ofatmospheric neutrinos is always accompanied by muons. Ifthese interactions with the atmosphere take place in theSouthern Hemisphere, the muons can penetrate the layer ofice above the detector and emit a signal that allows them to beidentified as coming from a spurious source. If they are ofatmospheric origin, the events detected by IceCube shouldmainly come from the Northern Hemisphere, from where themuons, absorbed by the Earth, cannot reach the detector andso it is possible to “suppress” the atmospheric source. On thecontrary, the IceCube events mainly cover the SouthernHemisphere. According to researchers, they unequivocallycome from outside the solar system.Investigations to study the direction of the source of the eventsand the time of their arrival have not yet allowed us to identifytheir sources: the number of detected neutrinos is still too lowand it is too soon to draw conclusions. Alternativeinterpretations involve the new physics, invoking for examplethe decay or self-annihilation of dark matter (see p. 25, ed.) oran increase in the neutrino collision cross-section or theintervention of leptoquarks, presumed mediators of theinteractions between quarks and neutrinos, predicted in someextensions of the standard model. These are just someexamples of the interesting options, old and new, which mightbe able to explain these observations by IceCube.The optical sensors continue to collect data, undisturbedbeneath the geographical South Pole and researchers hope toresolve this intriguing problem in the years to come.2013 was undoubtedly a decisive year for researchers atIceCube.

b.The neutrinos detected by IceCube maycome from astrophysical sources(cosmic or astrophysical neutrinos) orfrom interactions of cosmic rays withthe atmosphere (atmosphericneutrinos). IceCube “suppresses” thesource of atmospheric neutrinoscoming from the Southern Hemisphereby observing the muon produced incombination with the neutrino.



In addition to the discovery that signalled thebirth of astronomy of high-energy neutrinos,which has recently been confirmed, theinternational team published a measurementof neutrino oscillation parameters. Thesecharacterise the transformation of theneutrinos from one type (or flavour, which maybe electron, muon or tau) to another andconfirm that although their mass is very smallindeed, neutrinos are not massless. Theoscillation parameters reported by IceCubewere obtained by observing a reduction in theflux of atmospheric muon neutrinos in aparticular direction and at certain energies,using the most dense part of the detector’soptical sensors. This subunit, called DeepCore, is capable of recording neutrinos atlower energies, starting from a few GeV. Whatis relevant about this result is that itdemonstrates the potential for the use of neutrino telescopes in areas of research thatgo well beyond astrophysics, in the race forsuccess against competition from “targeted”experiments like Super-Kamiokande. And thatthey also hold some surprises. IceCube isalso searching for neutrinos emerging fromthe annihilation of dark matter (see page 27,ed.) that has accumulated due to thegravitational pull at the centre of celestialbodies such as Earth, the Sun and our galaxy.On the other hand, lowering the energythreshold (the minimum detectable energy ofa neutrino) even further to below 10 GeV, it

would become possible to measure the masshierarchy of neutrinos, meaning the order ofthe masses of the three neutrino flavours.With this aim, a new and bigger team ofresearchers are working on the PINGU projectto develop a dense core with another 40strings in IceCube. At high energies, work isbeing done to extend the sensitive volume ofIceCube by one order of magnitude (IceCube-Gen2) to discover the source of the recentlyglimpsed astrophysical signal.For neutrino telescopes this is just thebeginning of an exciting new era and newrevolutionary discoveries are presumably justaround the corner.

c.The IceCube laboratory at theAmundsen-Scott South PoleStation. The laboratory houses thecomputers that collect the data.Only events of interest are sent tothe University of Wisconsin -Madison, where they are analysedby physicists working on theproject.


BiographyElisa Bernardini is professor at the University of Humboldt in Berlin and at DeutschesElektronen-Synchrotron (DESY) in Germany. She is a member of the IceCube and MAGICresearch collaborations. Before moving to Germany in 2002, she studied in Bologna andthen in L’Aquila, in collaboration with the INFN’s Gran Sasso National Laboratory. Between2002 and 2005 she took part in three expeditions to the South Pole to carry outmaintenance on AMANDA, the forerunner of the IceCube neutrino telescope.

Web links

https://southpoledoc.wordpress.com/tag/icecube-neutrino-observatory/

http://www.km3net.org/home.php

http://baikalweb.jinr.ru


42

Fundamental chordsThe fascinating world of strings

by Marco Serone

Right now, the string theory is the most promising theory fortrying to resolve one of the biggest theoretical questions offundamental physics: how to integrate Einstein’s theory ofgravity (also known as the theory of general relativity) within theframework of quantum mechanics. String theory was originallydeveloped in 1968 for a completely different purpose (tounderstand strong interactions) by the Italian physicist GabrieleVeneziano. It is based on the assumption that all the elementaryparticles we can observe are nothing other than very smallvibrating strings. Just as the strings of a violin produce differentsounds depending on how they vibrate, so different oscillationsof strings correspond to different particles. Strings may beclosed or open and may even be tangled.Some years after Veneziano developed his theory, it becameclear that strong interactions are explained by a differenttheory, called quantum chromodynamics (QCD). Furthermore,at around the same time, researchers observed that the

different string vibrations always included the graviton, i.e.the particle responsible for gravitational interactions.Abandoned as the theory of strong interactions, the stringtheory thus acquired the more ambitious status of quantumtheory of gravity. Since then there have been severalimportant theoretical developments.String theory, whose structure is somewhat complex, seemsto effectively bring together a series of ideas circulating in thefield of fundamental physics to explain a number ofunanswered questions, especially theoretical ones, whichscientists face in the framework of the theory of generalrelativity and in the standard model of elementary particles.One of its main characteristics is the prediction according towhich in the universe there cannot only be the three spatialdimensions that we perceive (height, width and depth), butthere must be nine or ten dimensions, depending on whichvariant of the theory you are considering.


a.As the chords of the violin producedifferent sounds according to the wayin which they vibrate, similarly thedifferent oscillations of the stringscorrespond to the different particles.


We cannot perceive the six or seven extra dimensions becausethey are wrapped around themselves on the smallest of scales(physicists use the term compactified). To render the idea,imagine a very long and rather narrow tube. If observed from adistance, the tube looks like a one-dimensional object,practically a simple line, whereas when observed from close bythrough a lens or with a microscope, its two-dimensionalstructure is visible.Likewise, according to string theory, the extra dimensions areso subtle they cannot be observed. In order to discover theother dimensions, of which, incidentally, we do not know theexact length, we need an instrument that is much morepowerful than the current LHC accelerator.But why do we need these strings? Let me try to explain theproblem more clearly.The theory of general relativity states that space, time, energy,matter are all correlated, that is to say that spacetime, orrather its curvature, is determined by the energy and matter itcontains. The spacetime curve is what we perceive as gravity.The behaviour of matter over very short distances is insteadgoverned by quantum mechanics. Because gravity is by farthe weakest fundamental force that exists in nature, we candisregard the minimum space curvature induced by theelementary particles, when studying the infinitely small.Likewise, we can forget about the quantum mechanics ofelementary particles, when studying macroscopicphenomena. Thus, on one hand we have the theory of generalrelativity, which describes macroscopic gravitationalphenomena, on the other, there is quantum mechanics, whichdescribes the infinitely small and is the basis for all otherforces of nature.Both of these theories have been widely confirmed byexperiments, each within its own regime of validity (see. p.11, ed.), in which the others do not have a role. However,there are necessarily some energy and distance regimes inwhich you cannot describe a physical phenomenon using onlyone or the other theory. These regimes have not yet beendirectly explored, but at vastly smaller distances (or hugelyhigher energies) than those explored so far, gravity andquantum mechanics will inevitably have to be consideredtogether. Although we do not as yet have any physical process“at hand” that requires it, from a theoretical standpoint it isabsolutely essential that we understand how to combinegravity with quantum mechanics. Such a combination doesnot appear to be easy by any means. What makes stringtheory promising is the fact that, in a certain sense, itpredicts the very existence of gravity, since the vibration ofthe string that gives rise to the graviton is always present.On the other hand, our theory has a basic problem: at themoment, it is unclear what type of experiment could confirm orrefute its validity, and this is often a cause for criticism. But thevery fact that the theory comes into play in the extremeregimes of nature, which are not easily accessible, inevitablymakes its experimental verification difficult. It should also beadded that, regardless of the actual existence of strings, manytheories have been developed within this branch of researchand then migrated to the benefit of other fields of theoreticalphysics, so that, even only considering this aspect, the theorycan already be considered extremely fruitful.

In conclusion, since many aspects of strings have yet to beclarified, it would be premature to try to establish whether thetheory is just an extremely complicated (albeit very useful, atleast from a theoretical perspective) mathematical invention orwhether it is indeed the “theory of everything” that unifies allexisting fundamental physics within a single context.In any case, this is an ambitious concept without precedent inthe history of physics and, in the absence of any equally validalternative theories, the fascination and appeal of the stringtheory among theoretical physicists is entirely understandable.


BiographyMarco Serone is associate professor of Physics at the InternationalSchool for Advanced Studies in Trieste (SISSA). He is mainly concernedwith elementary particle physics and for several years was engaged inresearch in the field of the string theory.

Web links

http://www.stringwiki.org/wiki/String_theory_for_non-physicists

b.Two-dimensional projection of acompact space known as Calabi-Yau. This is a very popular type ofspace in the string theory. As canbe seen from the figure, the sixextra dimensions in these types ofspaces are generally rolled togetherin a very complicated way.


[as] intersections

They are us.by Kip Thorne

theoretical physicist and scientific adviser for the film Interstellar

Christopher Nolan’s recent film Interstellar is full of new physics.And that is no coincidence. Kip Thorne, a world-renowned expertin general relativity from the California Institute of Technology,agreed to act as scientific adviser for the film on condition thatthe scenes were consistent with present-day physical theories, orat least inspired by theoretical assumptions that are currently aresearch subject in physics: “educated guesses” that are notaltogether unfounded (ideas that could actually become new lawsof physics one day), or even outright speculations, but such thatscientists of his standing could regard as plausible. Thornehimself wrote a book (The Science of Interstellar, published byW.W. Norton & Company) that was released with the film. Anexcerpt is published below.

For the film, Thorne imagines something that many physicists haveactually suggested (see for example p. 42), namely that spacetimecontains additional space dimensions: it is what theoreticalphysicists refer to as a bulk, within which there is a sub-spacetime(physicists call this a brane, from “membrane”) with three spacedimensions and one time dimension, corresponding to thespacetime in which we live (see left of fig. a). For Interstellar,Thorne imagines that there is only one extra dimension: the bulk isthus composed of five dimensions, of which four space dimensionsand one time dimension. Strange beings, called “They” in the film,have managed to understand and overcome the gravitationalanomalies that affect the Earth. But who are “They” and howconvincing is all this? (Warning, this text contains spoilers!)

In 1844 Edwin Abbott wrote a satiricalnovella titled Flatland: A Romance ofMany Dimensions. Though its satire onVictorian culture seems quaint today andits attitude toward women outrageous,the novella’s venue is highly relevant toInterstellar. I recommend it to you. It describes the adventures of a square-shaped being who lives in atwo-dimensional universe called Flatland.The square visits a one-dimensionaluniverse called Lineland, a zero-dimensional universe called Pointland,and most amazing of all to him, a three-dimensional universe called Spaceland.And, while living in Flatland, he is visitedby a spherical being from Spaceland. In my first meeting with ChristopherNolan, we were both delighted to findthe other had read Abbott's novella andloved it.In the spirit of Abbott's novella, imaginethat you are a two-dimensional being, likethe square, who lives in a two-dimensional universe like Flatland. Youruniverse could be a tabletop, or a flat

sheet of paper, or a rubber membrane. Inthe spirit of modern physics, I refer to itas a two-dimensional (2D) brane. Being well educated, you suppose thereis a 3D bulk, in which your brane isembedded, but you’re not certain.Imagine your excitement when one dayyou are visited by a sphere from the 3Dbulk. A “bulk being”, you might call him. At first you don't realize it's a bulk being,but after much observation and thought,you see no other explanation. What youobserve is this: Suddenly, with nowarning and no apparent source a bluepoint appears in your brane. It expands tobecome a filled blue circle that expandsto a maximum diameter, then graduallyshrinks to a point and disappearscompletely (see right of fig. a). […]If there are bulk beings, what are theymade of? Certainly not atom-basedmatter like us. Atoms have three spacedimensions. They can only exist in threespace dimensions, not four. And this istrue of subatomic particles as well. Andit is true also of electric fields and

magnetic fields and the forces that holdatomic nuclei together. Some of the world’s most brilliantphysicists have struggled to understandhow matter and fields and forces behaveif our universe really is a brane in ahigher dimensional bulk. Those struggleshave pointed rather fìrmly to theconclusion that all the particles and allthe forces and all the fields known tohumans are confined to our brane, withone exception: gravity and the warping ofspacetime associated with gravity. There might be other kinds of matter andfields and forces that have four spacedimensions and reside in the bulk. But ifthere are, we are ignorant of their nature.We can speculate. Physicists dospeculate. But we have no observationalor experimental evidence to guide ourspeculations. In Interstellar, on ProfessorBrand’s blackboard, we see himspeculating. It’s a reasonable, half-educated guessthat, if bulk forces and fields andparticles do exist, we will never be able



to feel them or see them. When a bulk beingpasses through our brane, we will not see thestuff of which the being is made. The being’scross sections will be transparent. On the other hand, we will feel and see thebeing’s gravity and it’s warping of space andtime. For example, if a hyperspherical bulkbeing appears in my stomach and has astrong enough gravitational pull, my stomachmay begin to cramp as my muscles tighten,trying to resist getting sucked to the center ofthe being’s spherical cross section. […]All the characters in Interstellar are convincedthat bulk beings exist, though they use thatname only rarely. Usually, the characters callthe bulk beings “They”. A reverential They.Early in the movie, Amelia Brand says toCooper, “And whoever They are, They appearto be looking out for us. That wormhole letsus travel to other stars. It came along right aswe needed it”. One of Christopher Nolan’s clever andintriguing ideas is to imagine that They areactually our descendants: humans who, in thefar future, evolve to acquire an additionalspace dimension and live in the bulk. Late inthe movie, Cooper says to TARS, “Don’t youget it yet, TARS? They aren’t beings. They’reus, trying to help, just like I tried to helpMurph”. TARS responds, “People didn't buildthis tesseract” (in which Cooper is riding).“Not yet”, Cooper says, “but one day. Not youand me but people, people who’ve evolvedbeyond the four dimensions we know”.

Cooper, Brand, and the crew of the Endurance never actually feel or see ourbulk descendants’ gravity or their space warps and whirls. That, if it ever occurs, is left for a sequel to Interstellar. But older Cooperhimself, riding through the bulk in the closing tesseract, reaches out to theEndurance’s crew and his younger self, reaches out through the bulk, reachesout gravitationally. Brand feels and sees his presence, and thinks he is They.

Copyright © 2014 by Kip Thorne.With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.

a.Schematic diagram of a two-dimensional brane immersed in athree-dimensional bulk. On theright, a “bulk being” passingthrough the brane.


b.From left to right, David Gyasi (whointerprets the astronaut Romilly),Kip Thorne, Anne Hathaway(Amelia Brand in the film), JessicaChastain (Murphy Cooper), MichaelCaine (professor Brand) andStephen Hawking (friend of KipThorne, who’s life recently hasbeen showed in the film The Theoryof Everything) during the preview ofInterstellar in London.


[as] roots

The new, a hundred years ago.by Giuseppe Giuliani

physics historian

The expression “new physics”, which is the name given todayto the set of theories and phenomena that go beyond thestandard model of fundamental interactions, was first usedabout a century ago.In the late nineteenth century, mechanics, thermodynamicsand electromagnetism constituted the theoretical foundationsof physics. Within just a few years, the study of electricalconduction in rarefied gases, which began in the mid-nineteenth century, gave rise, directly or indirectly, to a numberof important discoveries: X-rays (1895), natural radioactivity(1896), the electron (1897). It was with reference to thesephenomena that Augusto Righi, the most famous Italianexperimental physicist of the time, coined the term “The NewPhysics” for the title of a conference held in 1912 at the ItalianSociety for the Advancement of Science (Società Italiana per ilProgresso delle Scienze - SIPS). These annual meetings werean important opportunity for interdisciplinary discussion anddissemination of the results of scientific research.

Starting from the early 1900s there were a host of newdiscoveries in physics. In 1900, Max Planck introduced the“constant of nature” h (later designated with his name) in asuccessful – though not altogether precise (as Einstein arguedin 1906) – attempt to explain the radiation in a hollow bodywhen thermal equilibrium is attained (“black body” radiation),later (1905-1907) interpreted by Einstein as the source ofdiscontinuity in the distribution of energy in different physicalsystems: beams of light and atoms that oscillate in crystalsaround their equilibrium positions. According to Einstein, light,under certain conditions, could be described as consisting of“light quanta” (later called photons), whose energy was linked,through Planck’s constant (h), to the frequency of lightdescribed as an electromagnetic wave.In 1911, Ernest Rutherford showed that atoms, in addition toelectrons, contain a positively charged nucleus. In 1913, NielsBohr, on the basis of the atomic model of Rutherford andassuming (with the use of the constant h) that the energy of


a.Augusto Righi coined the term “newphysics” in 1912, referring to thediscoveries of the late nineteenthand early twentieth centuries.


the electron of the hydrogen atom could only assume discretevalues, found an explanation for the electromagnetic radiationemitted or absorbed by the atom, surprisingly in agreementwith the experimental data obtained some time earlier. In themeantime (1912), Max von Laue demonstrated that crystallinesolids act as diffraction gratings for X-rays, thus opening theway for experiments to study crystalline structures. Incidentally,this technique eventually led to the discovery of the DNAstructure (1953).In parallel, Einstein worked on Newtonian dynamics and gravityto develop, respectively, the special theory of relativity (1905)and general relativity (1916).This led to a thorough review of the concept of matter: atoms,regarded as a heuristic hypothesis in the nineteenth century,became the subject of direct experimental and theoretical study.In the following decades, quantum mechanics, quantumelectrodynamics and quantum chromodynamics emerged as

powerful tools in theoretical investigations of atomic andsubatomic phenomena. Knowledge about the microscopic worldalso opened new avenues for understanding cosmic phenomenaand drafting a model to explain the origin of the universe.Since the end of World War II, experimental research hasincreasingly gone hand in hand with technology to the pointthat we can now speak of “techno-science”: scientificinvestigation relies on the products of technology, which are, inturn, fuelled by new knowledge. Alongside problems associatedwith the need for social control over technology are those dueto the persistence of irrational beliefs, the legacy of pastcenturies, that constantly re-ignite antiscientific attitudes.Scientists too must take part in this cultural battle bydisseminating their knowledge and promoting what, a centuryago, when “new physics” was born, Vito Volterra, founder ofthe SIPS and some of the most important scientific institutionsin Italy, called “scientific sentiment”.


b.Max Planck (left) awards the “MaxPlanck medal” of the GermanSociety of Physics to Albert Einsteinin Berlin on June 28, 1929.



Browsing information about the Higgs boson or the searchfor new physics instead of the latest photos of friends’holidays? Nothing could be simpler, if you have downloadedthe new The Particle Adventure app available free since lastNovember for iOS and Android.The history of The Particle Adventure goes back a long way.It was in 1989 when the first version was written usingHyperCard, a program for writing hyper texts before theadvent of the World Wide Web. In an era when the word“neutrino” was almost only heard among specialists and theHiggs boson had not yet filled the front pages ofnewspapers around the world, The Particle Adventure wasthe first attempt to make the standard model of particlesand the knowledge that had led to its formulation accessibleto a wide audience. In 1995 the program was converted intoa website, http: //www.particleadventure.org/, whichreceived nearly five million visits in the first year and hascontinued to be among the sites most frequently visited bystudents and onlookers from all over the world. The websiteis available in various languages.

The creation of the app (funded by the US EnergyDepartment and for now only available in English)is another step in spreading the fundamental ideas ofphysics and will also be very useful for students studyingmodern physics in the last year of high school.The most important discoveries and ideas of recentdecades, from quarks to the Higgs boson, from neutrinos tothe unification of fundamental forces, are organised in fivemain courses: “The Standard Model”, “Accelerators andparticle detectors”, “Higgs boson discovered”, “Exploringunsolved Mysteries” and “Particle decays and annihilations”.The contents of the app were developed by physicists withexpertise in various sectors, while the design, graphics andespecially the humorous parts were developed by physicsstudents. And they appear to have done an excellent job, ifyou think that one teacher, who was very sceptical at firstabout teaching modern physics at high-school, said that TheParticle Adventure made him change his mind, because thecareful blend of humour, graphics and science caught thestudents’ attention and kept them happily clicking on theirsmartphones. [Barbara Sciascia]

To download The Particle Adventure app:https://play.google.com/store/apps/details?id=gov.lbl.physics (Android)https://itunes.apple.com/us/app/the-particle-adventure/id924683946?ls=1&mt=8 (iOS)

[as] illuminations

Just a click away.


The laboratories of the Italian National Institutefor Nuclear Physics are open to visitors.

The laboratories organize free tours forschools and the general public on requestand by appointment.Visits last about three hours and includean introductory seminar on the activitiesof the INFN and the laboratoryand a tour of the experiments.

INFN laboratorycontact details:

Frascati National Laboratory (LNF) T + 39 06 94032423/ 2552 / 2643 / [email protected]

Gran Sasso National Laboratory (LNGS)T + 39 0862 [email protected]

Legnaro National Laboratory (LNL)T + 39 049 8068342 [email protected] www.lnl.infn.it

Southern National Laboratory (LNS)T + 39 095 [email protected]

www.infn.it

See also the websitewww.asimmetrie.it(only in Italian).

Asimmetrie is also an app,with lots of additional multimedia content(only in Italian).


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