CMS observes melting of Upsilon particles inheavy-ion collisions20 December 2012, by Achintya Rao, Cern

Candidate ? decay to two muons observed in a lead-leadcollision at the LHC. The two red lines (tracks) are thetwo muons, the mass of orange lines are tracks fromother particles produced in the collision, whose energy ismeasured in the electromagnetic calorimeter (redcuboids) and the hadron calorimeter (blue cuboids).

In 2011, CMS presented early evidence that Upsilon (?) particles produced in lead-leadcollisions "melt" as a consequence of interactingwith the hot nuclear matter created in these heavy-ion interactions. CMS has since updated andextended this result using additional data collectedin the 2011 heavy-ion run, and the observationnow has a significance of greater than 5? (or 5 standard deviations), the gold standard for claiminga discovery in high-energy physics.

Hot nuclear matter

The familiar proton is a composite particle made upof quarks held together by gluons. One of theproperties of these elementary quarks and gluonsis that they cannot exist freely and are alwaysbound together to form hadrons. At least not at thetemperatures we come across daily: Our bestunderstanding of the early Universe tells us that atthe very high temperatures prevalent in the firstmicroseconds following the Big Bang, quarks and

gluons existed freely as a homogenous mediumcalled the quark-gluon plasma (QGP).

Smashing heavy nuclei inside particle colliderscreates a hot nuclear matter thought to be similar tothe primordial QGP. Apart from colliding protons,the LHC dedicates beam-time to colliding leadnuclei (Pb) and the resulting collisions haveprovided physicists with a wealth of valuable datato study the hot nuclear matter. While it isn'tpossible to observe this medium directly, itspresence can be inferred based on the expectedbehaviour of the particles that interact with it.

Figure 1: The peaks in red show the measured ? eventsin PbPb collisions, with the blue lines showing theexpectations if the excited states were not suppressedwith respect to the ground state.

A literal melting pot

To explore the properties of the medium, CMScompares the properties of the particles producedin heavy-ion collisions (PbPb) with those producedin proton collisions (pp). A novel candidate forthese analyses is the ? particle, a meson made of abottom quark and its own antiparticle. Just as anatom can exist in an excited statean electron canreceive some energy and move to a higher

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orbitalso can composite particles such as the ?.The ?(1S) state is the ground state, while excitedstates include the ?(2S) and ?(3S) that are moreloosely bound than the ground state. A remarkableaspect of this measurement is that CMS is the firstexperiment to successfully separate the signals ofthe three ? states in heavy-ion collisions, thanks tothe superb mass resolution of the CMS detector.

Figure 2: The more central the PbPb collisions, thegreater the suppression. Events with more interactingnucleons (protons/neutrons) in the colliding nuclei indicating greater centrality in the collisions show ahigher suppression of ?s than peripheral collisions withfewer interacting nucleons.

Theorists predicted in the mid-'80s that having totraverse the hot medium will cause particles suchas ?s, produced in these interactions, to "melt" orbe suppressed in it. It is important to note that thissuppression doesn't mean that these particlesweren't produced in the first place, but that thebound states were possibly destroyed as a result ofinteracting with the medium before they coulddecay into muons. The more loosely bound thestate, the more easily it would melt; so, the (3S)state would be suppressed more than the (2S)state, while the (1S) state would be the leastsuppressed of the three.

Observing such a sequential suppression in thenumber of ? particles detected in collisions of leadnuclei relative to those in proton collisions at thesame energies helps probe the properties of thehot, dense medium.

The previous CMS analysis used just 7.3 ?b?1

(inverse microbarns) of PbPb data from 2010,whereas the new study uses 20 times as muchdata: 150 ?b?1 of PbPb collected in 2011. Since thelead-ion runs took place at a per-nucleon collisionenergy of 2.76 TeV and the proton run at the timewas at 7 TeV, the LHC provided a special pp run inearly 2011 at 2.76 TeV, with CMS collecting around230 nb?1 (inverse nanobarns) of data. CMSmeasured the decays of the three ? states into twomuons in both PbPb and pp collisions and foundthat the rates in the former were significantlysuppressed compared to the latter, as expected if aQGP were formed. The ratio of ?(2S) to ?(1S) inPbPb collisions was found to be approximately 21%that of the same ratio in pp collisions. Similarly, theratio of ?(3S) to ?(1S) was found to be only around6% in PbPb collisions relative to those with protons.

In addition, all three states were found to besuppressed in PbPb collisions. The suppression isfurther demonstrated by the fact that it is morepronounced in very central heavy-ioncollisionsones where the nuclei collide head-on(higher Npart in Figure 2)as opposed to peripheralcollisions. This behaviour is expected as centralcollisions release more energy and the hot nuclearmatter would consequently have a highertemperature, thereby melting more particles. Andbecause the more excited states melt at lowertemperatures, the sequential suppression is anexcellent indicator of the temperature of the nuclearmedium produced in PbPb collisions.

The 2011 results had a significance of 2.4?; thecurrent results are over 5? in significance, implyinga probability of less than one in 3.5 million ofobtaining a false positive. This observationrepresents an important milestone in the study ofheavy-ion collisions and has been made possibleby the excellent performance of the LHC itself aswell as collaborative efforts between the particle-physics (pp) and nuclear-physics (PbPb) groups atCMS. In early 2013, the LHC will smash protonswith lead nuclei, and the resulting cold nuclearmatter will help analyse the intricacies of theobserved sequential suppression.

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http://prl.aps.org/abstract/PRL/v109/i22/e222301http://prl.aps.org/abstract/PRL/v109/i22/e222301http://prl.aps.org/abstract/PRL/v109/i22/e222301http://cms.web.cern.ch/news/asymmetric-collisions-smashing-protons-lead-nucleihttp://cms.web.cern.ch/news/asymmetric-collisions-smashing-protons-lead-nuclei

More information:prl.aps.org/abstract/PRL/v109/i22/e222301physics.aps.org/articles/v5/132cds.cern.ch/record/1352773

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APA citation: CMS observes melting of Upsilon particles in heavy-ion collisions (2012, December 20)retrieved 19 March 2015 from http://phys.org/news/2012-12-cms-upsilon-particles-heavy-ion-collisions.html

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