particle physics 2

Bruce Kennedy, RAL PPD

Particle Physics 2

Bruce KennedyRAL PPD


Open questions

• What happened to the antimatter ?Why is there some matter left over

• What is the origin of mass ?Higgs mechanism (cf Bill Murray’s

talk)Can we find the Higgs particle ?

• Where does gravity come in ?“Theory of everything”


• Central idea in physics• A physical theory is defined by its symmetries• Simple eg: cos(x) = cos(-x)• More complex example:

QCD (theory of strong interaction) Invariant under “rotation” of quarks in “colour space”

• Symmetry described mathematically by Group Theory

Symmetries

Quantum Field Theory

Symmetry group

Standard Model

Particles

And

Forces

SU(3) x SU(2) x U(1)SO(10) ??


Where did the antimatter go ?

• … so it should all annihilate

Z0-

+

-

+

• Matter and antimatter created equallye.g.

… but there is some matter left over


u

_s

Matter-antimatter symmetry

• Symmetry operation “CP”P – parity – mirror reflection

(x,y,z) (-x,-y,-z)C – charge conjugation

particle antiparticle

• CP is an exact symmetry in physicse.g. rate for K++0 = K--0

• … except for neutral K & B mesons…

u

_s

_u

s

K+ K+K-


Symmetry breaking

• Decays of K0 and B0 are slightly different from anti-K0 and anti-B0

ONLY known matter-antimatter difference

Requires 3 quark-lepton generations

• Known as “CP-violation”• Effect is very small

Experimental study is difficult


The BaBar experiment

• Based at SLAC, Ca• Studies B mesons

>108 B-meson decays recorded

High-precision results

CP violation confirmed

Non-zero value

CP violation


Where is the Higgs particle ?

• Was it seen at LEP ? (see Bill Murray’s talk)

• How heavy is it ? At least 114 GeV No more than 1000 GeV (or 1 TeV)

• How can we find it (if it exists) Collide intense high-energy particle beams

(eg at LHC) Search for Higgs signature (not so easy…)


What about gravity ?

• Particle physics tries to unify forcesElectromagnetic+weak, strong

• Why not gravity ?• Symmetries of particle physics (SM)

and gravitation (GR) incompatibleCan be fixed by adding a new

symmetry“Supersymmetry” (SUSY)


• Particles exist asFermions (eg e, , q) – matter

particlesBosons (eg , Z, W) – force carriers

• In SUSY, fermions get boson partners (and vice versa)electron e ”selectron”photon “photino”

What is SUSY ?

SUSY


… so where are the SUSY particles ?

• Must be heavy… otherwise we would have found them SUSY is a “broken” symmetry

• How heavy ?No solid prediction from theoryProbably not more than 1 TeV

• Lightest SUSY particle should be stable(possible connection to Dark Matter)


• To study Higgs & supersymmetryNeed high energy beams

(particle masses up to 1000 GeV)… and very intense beams

(because interesting processes are very rare)

• New acceleratorThe Large Hadron Collider

proton-proton colliderBuilt in old LEP tunnelBeam energy 7 TeV, or 7000 GeVDue to start in 2007Accelerator and detectors now being built.

The Large Hadron Collider


LHC trivia

• 40 million collisions/sec• 1000 million pp interactions/sec

… but almost all of them are background

• Raw data rate is 1015 bytes/secequivalent to >1 million CD-roms/sec

• Only 0.00025% recorded for analysisexperimental “trigger” rejects the rest


Inside an LHC detector

ECAL

Tracker

HCAL

Magnet

Muon chambers


Finding the Higgs particle at LHC

• A few difficultiesWe don’t know the mass of the Higgs

Anywhere from 114 GeV to 1000 GeVDetection technique depends on mass

LHC produces 109 p-p interactions/sec… but only a few thousand Higgs/year

LHC is a proton-proton colliderSo not a clean environment like LEP


Finding SUSY particles at LHC

• Lightest SUSY particle leaves detector

• Detection relies on study of “missing” energy and momentum

• Seen in detector: 2 jets of “hadrons”

(mainly mesons) 2 muons 1 electron Missing energy

and momentum deduced from conservation laws.


What will we learn from LHC

• Should find “the” Higgs particleOr more than one ?

• Should discover supersymmetry(If it exists – no experimental evidence

so far)• Better understanding of CP violation

(Matter-antimatter differences)• Maybe something unexpected ?


What do we do next ?

• LHC good for “discovery”Need a more precise tool for detailed

understanding

• Muon collider ?Exciting prospect, but very difficult

• e+e- linear collider ?Europe, USA, Japan all have plans


Conclusion

• Exciting times ahead for particle physicsMatter-antimatter

Why is the universe made of matter ?Current experiments should give some answers

LHC should go beyond the Standard ModelHiggs particle(s), SUSY, new questions

New colliders planned for next generation of experiments


The CMS detector


The ATLAS detector


The LHCb detector


The ALICE detector


Example of a detector - CMS ECAL


LHC Detectors

ATLAS

LHCb

ALICECMS


Where to look for the Higgs ?

• Best method depends on its mass

• If it is light, we can look for decay to two photons


Underlying events

Simulated data


Brookhaven (USA) muon collider

• Muon lifetime is 2sNeed to

collectacceleratecollide

beams before they decay


TESLA linear collider (Germany)

• e+e- colliderLinear –

avoids radiation losses

33 km longEnergy up to

800 GeV

particle physics 2

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