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Particle Physics 2
Bruce Kennedy
RAL 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
Symmetries
• Central idea in physics
• A physical theory is defined by its
symmetries
Quantum
• Simple
eg: cos(x) = cos(-x)
Field
SU(3)
SO(10)
x SU(2)
?? x U(1)
Symmetry
group
• MoreTheory
complex example:
Particles
And
 QCD (theory of strong interaction)
 Invariant under “rotation” of quarks in “colour
space”
Forces
• Symmetry described mathematically
byUnification
Grand
Standard
Model
Group Theory
Bruce Kennedy, RAL PPD
Where did the antimatter go ?
• Matter and antimatter created equally
-
 e.g.
Z0
+
• … so it should all annihilate
-

+
…
but there is some matter left over
Bruce Kennedy, RAL PPD
Matter-antimatter symmetry
• Symmetry operation “CP”
P
– parity – mirror reflection
(x,y,z)  (-x,-y,-z)
C
– charge conjugation
K+
u
_
s
particle  antiparticle
• CP is an exact symmetry in physics
 e.g.
rate for K++0 = K--0
• … except for neutral K & B mesons…
Bruce Kennedy, RAL PPD
KK+
_
u
u
_
s
s
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 forces
 Electromagnetic+weak,
strong
• Why not gravity ?
• Symmetries of particle physics (SM) and
gravitation (GR) incompatible
 Can
be fixed by adding a new symmetry
 “Supersymmetry” (SUSY)
Bruce Kennedy, RAL PPD
What is SUSY ?
• Particles exist as
(eg e, , q) – matter particles
 Bosons (eg , Z, W) – force carriers
 Fermions
• In SUSY, fermionsSUSY
get boson partners
(and vice versa)
e  ”selectron”
 photon   “photino”
 electron
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 theory
 Probably not more than 1 TeV
• Lightest SUSY particle should be stable
 (possible
connection to Dark Matter)
Bruce Kennedy, RAL PPD
The Large Hadron Collider
• To study Higgs & supersymmetry
 Need
•
high energy beams
proton-proton
collider
(particle
masses
up to 1000 GeV)
Built in old LEP tunnel
 … and very intense beams
Beam energy 7 TeV, or 7000 GeV
(because
Due to startinteresting
in 2007 processes are very rare)
and detectors now
NewAccelerator
accelerator
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/sec
 equivalent
to >1 million CD-roms/sec
• Only 0.00025% recorded for analysis
 experimental
“trigger” rejects the rest
Bruce Kennedy, RAL PPD
Inside an LHC detector
HCAL
Muon
chambers
Tracker
ECAL
Magnet
Bruce Kennedy, RAL PPD
Finding the Higgs particle at LHC
• A few difficulties
 We
don’t 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
Bruce Kennedy, RAL PPD
Finding SUSY particles at LHC
• Seen in detector:
2
jets of “hadrons”
(mainly  mesons)
 2 muons
 1 electron
 Missing energy and
momentum deduced
from conservation
laws.
• Lightest SUSY
particle leaves
detector
• Detection relies on
study of “missing”
energy and
momentum
Bruce Kennedy, RAL PPD
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 ?
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 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
Bruce Kennedy, RAL PPD
Bruce Kennedy, RAL PPD
The CMS detector
Bruce Kennedy, RAL PPD
The ATLAS detector
Bruce Kennedy, RAL PPD
The LHCb detector
Bruce Kennedy, RAL PPD
The ALICE detector
Bruce Kennedy, RAL PPD
Example of a detector - CMS ECAL
Bruce Kennedy, RAL PPD
LHC Detectors
ATLAS
LHCb
ALICE
CMS
Bruce Kennedy, RAL PPD
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
Bruce Kennedy, RAL PPD
Brookhaven (USA) muon collider
• Muon lifetime is 2s
 Need
to
collect
accelerate
collide
 beams
decay
before they
Bruce Kennedy, RAL PPD
TESLA linear collider (Germany)
• e+e- collider
 Linear
– avoids
radiation losses
 33 km long
 Energy up to
800 GeV
Bruce Kennedy, RAL PPD
Symmetries
•
•
•
•
Central idea in physics
A physical theory is defined by its symmetries
Simple eg: cos(x) = cos(-x)
Particles
Quantum
MoreField
complex example:
And
SU(3)
SO(10)
x SU(2)
?? x U(1)
Symmetry
group
 QCD
(theory
of
strong
interaction)
Theory
 Invariant under “rotation” of quarks in “colour space”
Forces
• Symmetry described mathematically by
Group
Theory
Standard Model
Bruce Kennedy, RAL PPD