bruce kennedy, ral ppd particle physics 2 bruce kennedy ral ppd
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Bruce Kennedy, RAL PPD
Particle Physics 2
Bruce KennedyRAL PPD
Bruce Kennedy, RAL 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”
Bruce Kennedy, RAL PPD
• 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) ??
Bruce Kennedy, RAL PPD
Where did the antimatter go ?
• … so it should all annihilate
Z0-
+
-
+
• Matter and antimatter created equallye.g.
… but there is some matter left over
Bruce Kennedy, RAL PPD
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-
Bruce Kennedy, RAL PPD
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
Bruce Kennedy, RAL PPD
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
Bruce Kennedy, RAL PPD
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…)
Bruce Kennedy, RAL PPD
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)
Bruce Kennedy, RAL PPD
• 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
Bruce Kennedy, RAL PPD
… 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)
Bruce Kennedy, RAL PPD
• 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
Bruce Kennedy, RAL PPD
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
Bruce Kennedy, RAL PPD
Inside an LHC detector
ECAL
Tracker
HCAL
Magnet
Muon chambers
Bruce Kennedy, RAL PPD
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
Bruce Kennedy, RAL PPD
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.
Bruce Kennedy, RAL PPD
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 ?
Bruce Kennedy, RAL PPD
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
Bruce Kennedy, RAL PPD
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
Bruce Kennedy, RAL PPD
Bruce Kennedy, RAL PPD
The CMS detector
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The ATLAS detector
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The LHCb detector
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The ALICE detector
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Example of a detector - CMS ECAL
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LHC Detectors
ATLAS
LHCb
ALICECMS
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Where to look for the Higgs ?
• Best method depends on its mass
• If it is light, we can look for decay to two photons
Bruce Kennedy, RAL PPD
Underlying events
Simulated data
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Brookhaven (USA) muon collider
• Muon lifetime is 2sNeed to
collectacceleratecollide
beams before they decay
Bruce Kennedy, RAL PPD
TESLA linear collider (Germany)
• e+e- colliderLinear –
avoids radiation losses
33 km longEnergy up to
800 GeV