Particle Physics 2

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Particle Physics 2

• Bruce Kennedy
• RAL PPD

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Open questions

• What happened to the antimatter ?
• Why is there some matter left over
• What is the origin of mass ?
• Higgs mechanism (cf Bill Murrays talk)
• Can we find the Higgs particle ?
• Where does gravity come in ?
• Theory of everything

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Symmetries

• 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

Particles And Forces
Quantum Field Theory
Symmetry group
SU(3) x SU(2) x U(1)
SO(10) ??
Standard Model
Grand Unification
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Where did the antimatter go ?

• Matter and antimatter created equally
• e.g.

?-

• so it should all annihilate

Z0
?
?-
?
?

• but there is some matter left over

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Matter-antimatter symmetry
K
K-
K

• Symmetry operation CP
• P parity mirror reflection
• (x,y,z) ? (-x,-y,-z)
• C charge conjugation
• particle ? antiparticle
• CP is an exact symmetry in physics
• e.g. rate for K???0 K-??-?0
• except for neutral K B mesons

_u s
u _s
u _s
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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

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The BaBar experiment

• Based at SLAC, Ca
• Studies B mesons
• gt108 B-meson decays recorded
• High-precision results
• CP violation confirmed

Non-zero value ? CP violation
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Where is the Higgs particle ?

• Was it seen at LEP ?
• (see Bill Murrays 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)

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What about gravity ?

• Particle physics tries to unify forces
• Electromagneticweak, strong
• Why not gravity ?
• Symmetries of particle physics (SM) and
  gravitation (GR) incompatible
• Can be fixed by adding a new symmetry
• Supersymmetry (SUSY)

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What is SUSY ?

• Particles exist as
• Fermions (eg e, ?, q) matter particles
• Bosons (eg ?, Z, W) force carriers
• In SUSY, fermions get boson partners (and vice
  versa)
• electron e ? selectron
• photon ? ? photino

SUSY
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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 theory
• Probably not more than 1 TeV
• Lightest SUSY particle should be stable
• (possible connection to Dark Matter)

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The Large Hadron Collider

• To study Higgs supersymmetry
• Need high energy beams
• (particle masses up to 1000 GeV)
• and very intense beams
• (because interesting processes are very rare)
• New accelerator
• The 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.
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LHC trivia

• 40 million collisions/sec
• 1000 million pp interactions/sec
• but almost all of them are background
• Raw data rate is 1015 bytes/sec
• equivalent to gt1 million CD-roms/sec
• Only 0.00025 recorded for analysis
• experimental trigger rejects the rest

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Inside an LHC detector
HCAL
Muon chambers
Tracker
ECAL
Magnet
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Finding the Higgs particle at LHC

• A few difficulties
• We dont know the mass of the Higgs
• Anywhere from 114 GeV to 1000 GeV
• Detection technique depends on mass
• LHC produces 109 p-p interactions/sec
• but only a few thousand Higgs/year
• LHC is a proton-proton collider
• So not a clean environment like LEP

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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.

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What will we learn from LHC

• Should find the Higgs particle
• Or 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 ?

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What do we do next ?

• LHC good for discovery
• Need a more precise tool for detailed
  understanding
• Muon collider ?
• Exciting prospect, but very difficult
• ee- linear collider ?
• Europe, USA, Japan all have plans

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Conclusion

• Exciting times ahead for particle physics
• Matter-antimatter
• Why is the universe made of matter ?
• Current experiments should give some answers
• LHC should go beyond the Standard Model
• Higgs particle(s), SUSY, new questions
• New colliders planned for next generation of
  experiments

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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
ALICE
CMS
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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

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Underlying events
Simulated data
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Brookhaven (USA) muon collider

• Muon lifetime is 2?s
• Need to
• collect
• accelerate
• collide
• beams before they decay

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TESLA linear collider (Germany)

• ee- collider
• Linear avoids radiation losses
• 33 km long
• Energy up to 800 GeV

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Symmetries

• 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

Particles And Forces
Quantum Field Theory
Symmetry group
SU(3) x SU(2) x U(1)
SO(10) ??
Standard Model