LHC Machine and Detectors

1
LHC - Machine and Detectors

• Christoph Rembser
• CERN, Switzerland

Lecture 1

• Basic concepts of accelerator and detectors
• Design / status of machine and experiments
• Experiences (sometimes painful), failures,
• problems (solved, un-solved)
• Personal motivation, lessons learned

Non scholae sed vitae (discimus)
2
Physics at colliders is exiting

• End of the ee- collider LEP at CERN

Higgs candidates observed
Most significant ALEPH event
3
for the physicists
interested in details? ?CR CD
4
and for public
5
LHC - the schedule imagined in 1990
6
LHC - the schedule imagined in 2000
Official Realistic

• Should have started already, but LHC is a
  challenge ? this talk

7
Which energies we need?

• Search for the Higgs particle
• LEP
• Theory/experiments
• Search for SUSY
• No hints for SUSY particles so far expect new
  physics lt 1 TeV

Mhiggs gt 114.1 GeV _at_ 95 C.L. (Mhiggs 115.0
(1.3/-0.9) GeV (2?)
aiming for discoveries!!!
Mhiggs 118 (63/-42) GeV Mhiggs lt 236 GeV _at_ 95
C.L.
? Explore energies up to few TeV
How to reach these energies?
8
Particle accelerators towards highest energies

• use electric fields to accelerate particles
• use magnetic fields to steer and focus particle
  beams
• ammunition for accelerators charged stable
  particles
• electrons, protons, ions
• steer beams onto
• target (fixed target operation)
• ? for hadrons
  _at_ 450GeV
• second beam (collider operation)
• ? for hadrons
  _at_ 450GeV

How should the accelerator look like, which
particle to choose?
9
Acceleration in an electric field

• modern accelerators work with time varying
  fields. example linear accelerator
• bunched beam ?
• long accelerator for high energies ?
• particles lost after collision ?

10
Example of a linac
AC voltage
A small linac (GSI, ions _at_ 1MeV)
11
Phase stability
late (slow) particle high field, more accel.
particle in time, optimum
vltltc
early (fast) particle low field, less accel.
early particle low momentum ? smaller
orbit high field, more accel.
vc
particle in time, optimum
late particle high momentum ? bigger
orbit low field, less accel.
focussing effect, but needs phase shift (17GeV _at_
Tevatron)
12
Circular accelerator
Multiple passage through accelerating unit

• reaches higher energies
• allowes storage of beam
• Needs bending magnet(s)

standard (warm) magnets fields up to 2 T
13
Collisions of particles

• Possibilities for collisions with circular
  accelerators

• ? Collide particles(anti)-particles
• 2 x (beam pipes beam steering)
• - more expensive
• higher collision rate possible
• (?this lecture)

• ? Collide particlesantiparticles
• 1 x (beam pipe beam steering)
• cheaper

14
Synchrotron radiation

• Electromagnetic energy emitted by accelerated
  charged particles
• Slow particles (classical Ansatz, Lamor)
• Relativistic particle (Lorentz
  transformation)

Forwards, backwards
Forward boosted
15
Synchrotron Radiation p or e?

• Emitted power

U(loss per turn)?4/?
?4(E/m)4 ? Pe 2 ?1012 x Pp (at identical
particle energies)
? some examples
Electron machine
Proton machine
? prefer proton machine, less power losses
16
Proton machines are discovery machines!
t
W, Z
Upsilon (bb) (400GeV p-nucleon, Fermilab)
J/psi (cc) (AGS proton Synchrotron Brookhaven)
17
Which energy (a bit more precise)

• Requiredproduce new particles up to 1 TeV

Relevant centre-of-mass energy ?s of colliding
constituents (q, g) Rough estimate ?s 1/2 ?
1/3 ? ?s Need ?s 6 ? 2 ? 1 TeV ? 14 TeV ( 7
TeV/beam)
x fraction of p momentum carried by the
colliding partons u, d, g x up to 0.35 ? LHC
mass limit for new particles 5 TeV
18
Size of accelerator

• Keep power P emitted by synchrotron radiation
  low

1/?2 (? ring radius) ? build huge facilities!

• Keep field B of bending magnets small

1/? ? build huge facilities!
Huge accelerator facilities are expensive ? re-
use existing infrastructure (?)
19
A new collider in the LEP Tunnel
1984!
2p? 27km
20
or in the US near Dallas?

• Super superconducting collider SSC
• Waxahachie/Texas
• Proton machine
• 20 TeV/beam
• 2??87.1km
• 1983-1993

21
Advantage CERN infrastructure(1)
22
Advantage CERN infrastructure(2)

• Pre-accelerators available (injection into LHC at
  450GeV from SPS)
• Tunnel ready
• 50 years of experience

23
More on accelerators luminosity

• the possibility of an interaction / cm2 /s

with Ni number of particles in bunch I f
number of bunch crossings /s A area of
beam which interacts
24
Event rate

• Actual collision rate depends on the effective
  size ?of the colliding objects

total inelastic cross section
background
? 10-25 cm2 (strong)
point like cross section
? 10-37 cm2 (electroweak)
signal
25
Particle production rates _at_14TeV
? Inelastic p-p reaction ? 1 billion / s ? b
pairs ? 1 million / s ? t pairs ? 8 / s ? W? e?
? 150 / s ? Z ? ee ? 15 / s ? Higgs ? 0.2 / s ?
gluino, squark ? 0.03 / s

• ?1pb

26
How to reach 1034 / cm2 s ?

• Luminosity

? Increase number of particles in bunches
(Ni) LHC 1.151011 (upgrade 1.71011, only 2
experim. zones)

• Go for many bunches (nbunches)
• LHC fill 2808 bunches
• Consequenz bunch crossing each 25ns very fast

Limitations - radiation issues and damage
potential - charged particles
? beam spreads out - heat load
due to electron cloud effect ?
27
Electron cloud limitation

• Synchrotron light removes electrons from chamber
  wall
• Electrons are accelerated by beam
• Electrons hit vacuum chamber, produce secondaries

LHC - first hadron collider affected by SR!!!
? Instability and heat loss at cryogenic
temperatures. Worse for 12.5ns bunch spaceing
(upgrade option)
28
How to reach 1034 / cm2 s (2)?

• Luminosity

? Decrease beam size A, A 4??x?y with ?i
v (?i ?i)

• ? betatron oszillation, determined by focussing
  quadrupole
• ? emittance, measurement of the beam divergency
• (beam size(x, y)
  beam spread (px, py))
• ?n / ? with ?n is normalized emittance,
• determined by
  pre-accelerators

? const. (Liouville)

• ? reduce ? at interaction point
• ? decrease emittance at injector chain and
• increase collision energy

29
Beam size at interaction point

• Best collinde bunches head-on, no angle between
  beam directions
• ? many beam-interactions before reaching IP
• At the interaction point, ? is squeezed from
  100m to 0.5m ? crossing angle of 300 ?rad

still 15 long-range interactions of each side
of IP
30
Reduce emittance (beam damping)

• Beam size shrinks during acceleration?
• Ideal beam, no disturbance, optimal orbit s
• N.B. no other efficient damping at high energies
  for hadron colliders
• ? damping at pre-accelerators is important!
• ionisation cooling
• stochastic cooling

Acceleration p??s increases, p?s stays
const. ? p bents toward s (acceleration
damping)
?