Introduction to CERN Activities
•Intro to particle physics
•Accelerators – the LHC
•Detectors - CMS
Introduction to CERN
David Barney, CERN
From atoms to quarks I
Introduction to CERN
David Barney, CERN
From atoms to quarks II
Leptons are fundamental
e.g.
electron
muon
neutrinos
Hadrons are made of quarks,
e.g.
p = uud
Baryons
L0 = uds
L0b = udb
p+ = ud
Mesons
Y = cc
U = bb
Introduction to CERN
David Barney, CERN
The structure of the Proton
Proton is not, in fact, simply
made from three quarks (uud)
There are actually 3 “valence”
quarks (uud) + a “sea” of gluons
and short-lived quark-antiquark
pairs
Introduction to CERN
David Barney, CERN
Matter and Force Particles
Leptons
Strong
Electromagnetic
Electric Charge
Tau
-1
0
Tau
Neutrino
Muon
-1
0
Muon
Neutrino
Electron
-1
Electron
Neutrino
0
Quarks
Gluons (8)
Photon
Quarks
Mesons
Baryons
Nuclei
Atoms
Light
Chemistry
Electronics
Weak
Gravitational
Electric Charge
Bottom
Strange
Down
-1/3 2/3
Top
-1/3 2/3
Charm
-1/3 2/3
Up
each quark: R,
Introduction to CERN
B,
Graviton ?
Solar system
Galaxies
Black holes
G 3 colours
The particle drawings are simple artistic representations
Bosons
(W,Z)
Neutron decay
Beta radioactivity
Neutrino interactions
Burning of the sun
David Barney, CERN
Characteristics of the 4 forces
What characterizes a force ? Strength, range and source charge of the field.
Interaction
Exchanged
quantum
(source ch)
Range
(m)
Relative
Strength
Examples
in nature
gluon
10-15
1
proton (quarks)
colour
Electromagnetic
photon
<10-2
atoms
electric
Weak
W, Z
<10-17 10-5
radioactivity
hypercharge
Gravity
graviton ?
10-38
solar system
mass
Ratio of electrical to gravitational force between two protons is ~ 1038 !!
Can such different forces have the same origin ??
Strong
Introduction to CERN
David Barney, CERN
Unification of fundamental forces
Magnetism
QED
Electro
magnetism
Quantum
Gravity
Grand
Unification
SUSY?
Electroweak
Model
Standard
model
Maxwell
Weak Theory
Long range
Electricity
Fermi
Weak Force
Short range
Nuclear Force
QCD
?
Short range
Super
Unification
Kepler
Universal
Gravitation
Einstein, Newton
STRINGS?
Introduction to CERN
Theories:
RELATIVISTIC/QUANTUM
Celestial
Gravity
Long range
Terrestrial
Galilei Gravity
CLASSICAL
David Barney, CERN
Unanswered questions in Particle Physics
a. Can gravity be included in a theory with the other three interactions ?
b. What is the origin of mass?  LHC
c. How many space-time dimensions do we live in ?
d. Are the particles fundamental or do they possess structure ?
e. Why is the charge on the electron equal and opposite to that on the proton?
f. Why are there three generations of quark and lepton ?
g. Why is there overwhelmingly more matter than anti-matter in the Universe ?
h. Are protons unstable ?
i. What is the nature of the dark matter that pervades our galaxy ?
j. Are there new states of matter at exceedingly high density and temperature?
k. Do the neutrinos have mass, and if so why are they so light ?
Introduction to CERN
David Barney, CERN
The Standard Model
Me ~ 0.5 MeV
Mn ~ 0
Mt ~ 175,000 MeV!
Mg = 0
MZ ~ 100,000 MeV
Why ?
Where is Gravity?
Introduction to CERN
David Barney, CERN
Mathematical consistency of the SM
WL
ZL
SM gives nonsense !
WL
time
ZL
WL
WL
At energies > 1 TeV the probability of scattering of
one W boson off of another becomes greater than 1
ZL
H
A popular solution is to introduce a Higgs
exchange to cancel bad high energy behaviour
WL
Introduction to CERN
David Barney, CERN
What is wrong with the SM?
• SM contains too many apparently arbitrary features
• SM has an unproven element - not some minor detail but a central element namely mechanism to generate observed masses of the known particles
a popular solution is to invoke the Higgs mechanism
• SM gives nonsense at high energies.
At centre of mass energies > 1000 GeV the probability of W LWL scattering
becomes greater than 1!
a popular solution is to introduce a Higgs exchange to cancel the bad high
energy behaviour
• SM is logically incomplete - does not incorporate gravity - build TOE
is superstring theory the TOE ?
Introduction to CERN
David Barney, CERN
Origin of mass and the Higgs mechanism
Simplest theory – all particles are massless !!
A field pervades the universe
Particles interacting with this field acquire mass –
stronger the interaction larger the mass
The field is a quantum field – the quantum is the Higgs
boson
Finding the Higgs establishes the presence of the field
Introduction to CERN
David Barney, CERN
CERN Site
LHC
SPS
CERN Site (Meyrin)
Introduction to CERN
David Barney, CERN
CERN Member States
Introduction to CERN
David Barney, CERN
CERN Users
Introduction to CERN
David Barney, CERN
Particle Collider
Circular accelerator
with colliding beams
Beam particles
Accelerating cavities
Deflection magnets
Focussing magnets
Particle source
Introduction to CERN
David Barney, CERN
Types of Particle Collider
Electron-Positron Collider (e.g. LEP)
e-
e+
Electrons are elementary particles, so
Proton-Proton Collider (e.g. LHC)
d
u
d
u
u
u
Eproton1 = Ed1 + Eu1 + Eu2 + Egluons1
Ecollision = Ee- + Ee+ = 2 Ebeam
Eproton2 = Ed2 + Eu3 + Eu4 + Egluons2
e.g. in LEP, Ecollision ~ 90 GeV
= mZ
Collision could be between quarks
or gluons, so
i.e. can tune beam energy so that
you always produce a desired particle!
Introduction to CERN
0 < Ecollision < (Eproton1 + Eproton2)
i.e. with a single beam energy you can
“search” for particles of unknown mass!
David Barney, CERN
CERN Accelerator Complex
Introduction to CERN
David Barney, CERN
Collisions at the Large Hadron Collider
7x1012 eV
1034 cm-2 s-1
2835
1011
Beam Energy
Luminosity
Bunches/Beam
Protons/Bunch
7.5 m (25 ns)
7 TeV ProtonProton
colliding beams
Bunch Crossing4x107 Hz
Proton Collisions 109 Hz
e-
Parton Collisions
New Particle Production
(Higgs, SUSY, ....)
µ+
105 Hz
p
µ+
Z
H
Introduction to CERN
c1-
µp
~
q
~
g
p
~
q
Z
q
µ-
ne
q
~
c20
q
p
m+
mc~1 0
David Barney, CERN
LHC Detectors
General-purpose
Higgs
SUSY
??
B-physics
CP Violation
Introduction to CERN
Heavy Ions
Quark-gluon plasma
General-purpose
Higgs
SUSY
??
David Barney, CERN
The two Giants!
ATLAS A Toroidal LHC ApparatuS
CMS Compact Muon Solenoid
µ
µ
Introduction to CERN
David Barney, CERN
Particle Detectors I
• Cannot directly “see” the collisions/decays
– Interaction rate is too high
– Lifetimes of particles of interest are too small
• Even moving at the speed of light, some particles (e.g. Higgs) may
only travel a few mm (or less)
• Must infer what happened by observing long-lived
particles
– Need to identify the visible long-lived particles
• Measure their momenta
• Energy
• (speed)
– Infer the presence of neutrinos and other invisible
particles
• Conservation laws – measure missing energy
Introduction to CERN
David Barney, CERN
Particle Momentum Measurement
• Electrically charged
particles moving in a
magnetic field curve
• Radius of curvature is
related to the particle
momentum
– R = p/0.3B
• Should not disturb the
passage of the particles
• Low-mass detectors
sensitive to the passage of
charged particles
• Many layers – join the dots!
• E.g. CMS silicon tracker
Introduction to CERN
Electron
In CMS
David Barney, CERN
Energy Measurement - Calorimeters
• Idea is to “stop” the
particles and measure
energy deposit
• Particles stop via energy
loss processes that
produce a “shower” of many
charged and neutral
particles – pair-production,
bremstrahlung etc.
• Detector can be to
measure either hadrons or
electrons/photons
Introduction to CERN
• Two main types of
calorimeter:
– Homogeneous: shower
medium is also used to
produce the “signal” that is
measured – e.g. CMS
electromagnetic calorimeter
– Sampling: the shower
develops in one medium,
whilst another is used to
produce a signal proportional
to the incident particle
energy – e.g. CMS Hadron
Calorimeter
David Barney, CERN
Particle interactions in detectors
Introduction to CERN
David Barney, CERN
CMS – Compact Muon Solenoid
Introduction to CERN
David Barney, CERN
CMS – Compact Muon Solenoid
Introduction to CERN
David Barney, CERN
Puzzle
Introduction to CERN
Find 4 straight tracks.
David Barney, CERN
Answer
Make a “cut” on the
Transverse momentum
Of the tracks: pT>2 GeV
Introduction to CERN
David Barney, CERN