pphysics today 2

Particle Physics 2
Bruce Kennedy
RAL PPD

Bruce Kennedy, RAL


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


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:
 QCD

(theory of strong interaction)

Particles
And
Forces

 Invariant under “rotation” of quarks in “colour
space”

• Symmetry described mathematically
by
Grand
Standard
Unification
Model
Group Theory

Bruce Kennedy, RAL


Where did the antimatter go ?
• Matter and antimatter created equally
µ
-

 e.g.

Z0

µ+

• … so it should all annihilate
µ-

µ

+

…

but there is some matter left over

Bruce Kennedy, RAL

γ

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+→π+π0 = K-→π-π0

• … except for neutral K & B mesons…
Bruce Kennedy, RAL

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


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


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


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


What is SUSY ?
• Particles exist as
(eg e, µ, q) – matter particles
 Bosons (eg γ, Z, W) – force carriers
 Fermions

SUSY
• In SUSY, fermions
get boson partners
(and vice versa)

e ↔ ”selectron”
 photon γ ↔ “photino”
 electron

Bruce Kennedy, RAL

… 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


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


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


Inside an LHC detector
HCAL
Muon
chambers

Tracker
ECAL

Magnet
Bruce Kennedy, RAL


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


Finding SUSY particles at LHC
• Seen in detector:
2

jets of “hadrons”
(mainly π 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


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


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


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


Bruce Kennedy, RAL PPD


The CMS detector

Bruce Kennedy, RAL


The ATLAS detector

Bruce Kennedy, RAL


The LHCb detector

Bruce Kennedy, RAL

The ALICE detector

Bruce Kennedy, RAL


Example of a detector - CMS ECAL

Bruce Kennedy, RAL