1. Accelerators and Detectors
An introduction
Barbro Åsman

2. Why accelerate particles?
Because:
They should probe an other particles
The high kinematic energy should be transformed
into mass according to:
E = mc2 e

3. The Discovery of the quarks
3 km lång accelerator

4. Linear Accelerator - + - + + + - + + - + +
High voltage

5. Charged particles in a magnetic field
Moving charged particles are affected by a magnetic field
R = P
B x q

6. Elektron i magnetfält
High velocity
low velocity

7. Cyklotron

8. Synkrotron
SPS
Tesla

9. Why are the accelerators so big?
The magnet fields can not be made too strong
B = p / r x q
Charged particles radiate synchrotron light when they accelerate.
Energy Loss per particle and turn :
DE =4pe2b3g4/(3xr)
r = bending radius ,
b = v/c ,
g = (1 – b2) -1/2 = E / m0c2

10. High energy laboratories
FNAL
DESY
CERN
BNL
KEK
SLAC

11. See particles ? We can determine: The particles direction
The type of particle
The particles energy

12. Detecting Particles

13. Electromagnetic Interaction of Particles with Matter
The incoming
particle loses
energy .
The atoms are
excited or ionized.
The particle is deflected multiple scattering
During scattering a photon can be emitted.
Bremstrahlung
The particle’s velocity larger than the velocity of light in
the medium, ->
Cherenkov Radiation.
The particle crosses the boundary between two media -> Transition radiation.

14. Bethe - Bloch re = classical electron radius = e2/(4pe0mec2 )
me = electron mass
NA = Avogadro's number
I = mean excitation energy
Z = atomic number
A = atomic weight
= density
z = charge of incoming particle
B = v/c of incoming particle
Wm= the maximum transferable
energy of the particle
d = “density – effect”
C = shell correction term

15. What is Wm ? Ek + mec2 = p´2c2 + me2c4 Homework ->
Scattered particle with
momentum p´´ q
Scattered electron with
momentum p´
Incoming particle with
momentum p
Ek + mec2 = p´2c2 + me2c4
Where Ek is the kinetic energy of the electron
Homework ->

16. What is Wm ? Maximum energy transfer is when q = 0
Wm = 2mec2b2g (me/m)2 + 2g(me/m) -1
me < m >
g >> m/me ->
g <<m/me ->
m = me >
Wm = 2mec2b2g2 2g(me/m) -1
Wm = mec2b2g2 / (1+g)
Wm = 2mec2b2g g(me/m) -1
Wm = 2mec2b2g2

17. -b2 comes from a quantum treatment of the
energy loss by collisions of heavy spin 0
particles
-d (g) is due to density effects, increases
with energy
-C/Z is due to non participation of inner
electrons. Small and often ignored.

18. A B C D
A rapid decrease following 1/b2
B minimum at about g = 3
C slow increase following ln(g )
D a plateau is reached due to density effects.
1) for low density materials like gases at g=1000
2) for solids g could be as low as 10

19.  A 1 GeV muon can traverse 1m of Iron
1/ dE/dx  1.4 MeV cm 2/g
for ßγ  3
Thickness = 100 cm;
ρ = 7.87 g/cm3
dE ≈ 1.4 * 100* 7.87 = 1102 MeV
 A 1 GeV muon can traverse 1m of Iron
1/
This number must be multiplied with ρ [g/cm3] of the Material  dE/dx [MeV/cm]

20. Ionization in emulsion
Kaon
Pion
Small energy loss
 Fast particle
Large energy loss
 Slow particle

21. Range of particle in matterB C D
Bragg Peak:
For >3 the energy loss is  constant
The energy of the particle falls below =3 the energy loss rises as 1/2

22. Energy Loss as a Function of the Momentum
The energy loss as a function of
particle momentum P= Mcβγ
By measuring the particle momentum
and measurement of the energy loss ->
measure the particle mass
 Particle Identification

23. Measure momentum by curvature of the particle track.
Find dE/dx by measuring the deposited charge along the track.
Particle ID

24. Energy Loss Fluctuations
X X X X X
XX X X
X X X
XXX X XX
X XXX X E
E -  
The energy loss is a statistical process and will therefore fluctuate from event to event.

25. Landau Distribution
P()
P(): Probability for energy loss  in matter of thickness D.
Landau distribution is very asymmetric.
Average and most probable energy loss must be distinguished !
Energy loss (arbitrary scale)

26. Electrons in Matter Ionization:
Electrons compared to heavier particles
they are always relativistic
they can lose all energy in one interaction
interaction of two identical particles has
to be allowed according to quantum mechanics

27. Electrons ionization in Matter
dE = 2Cmc2 Zr ln pg3/2mc2 – a/2 – d/2 – C/Z
dx A I
C = 2pNAre2
b = 1
z = 1
a = 2.9 for electron
a = 3.6 for positron
Heavy particle
dE = 2Cmc2 Zr ln 2g2mc2 – 1 – d/2 – C/Z
dx A I

28. Electromagnetic Interaction of Particles with Matter
The incoming
particle loses
energy .
The atoms are
excited or ionized.
The particle is deflected multiple scattering
During scattering a photon can be emitted.
Bremstrahlung
The particle’s velocity larger than the velocity of light in
the medium, ->
Cherenkov Radiation.
The particle crosses the boundary between two media -> Transition radiation.

29. Bremsstrahlung
Electron are accelerated in the field of the nuclei and radiate.
Passage through matter results in emission of high energy photons.

30. Bremsstrahlung small g large g
X0(cm): Distance where the Energy E0 of the incoming particle decreases E0 e-1

31. Electron in Matter

32. Critical Energy
Muon Momentum
Electron Momentum MeV/c
Critical Energy: If dE/dx (Ionization) = dE/dx (Bremsstrahlung)
Myon in Copper: p  400GeV
Electron in Copper: p  20MeV

33. Electromagnetic Interaction of Particles with Matter
The incoming
particle loses
energy .
The atoms are
excited or ionized.
The particle is deflected multiple scattering
During scattering a photon can be emitted.
Bremstrahlung
The particle’s velocity larger than the velocity of light in
the medium, ->
Cherenkov Radiation.
The particle crosses the boundary between two media -> Transition radiation.

34. Multiple scattering XFe = 2 cm XBe = 33 cm
Statistical analysis of multiple collisions gives:
The root mean square of the projected scattering angle distribution is given by:
x = the thickness of the material
X0 = radiation length of the material
Z1 . = charge of the incoming particle
bc = velocity of the incoming particle
p = momentum of the incoming particle
XFe = 2 cm
XBe = 33 cm

35. LEP Experimental Chambers
All 4 LEP experiments had beryllium vacuum chambers around the IP
Longest LEP chamber was OPAL Phase II
Each LEP chamber was removed and re-fitted ~5 times
LEP chambers were baked-out in the lab, not in-situ as for ALICE
Each LEP chamber was mechanically modified at least once
Comparison of OPAL and ALICE Dimensions
Inner diameter (mm)
Wall thickness (mm)
Be length (mm)
Total Length (mm)
OPAL Phase II
106
1.1
4500
6000
ALICE 58
0.8
3950
4750

36. Electromagnetic Interaction of Particles with Matter
The incoming
particle loses
energy .
The atoms are
excited or ionized.
The particle is deflected multiple scattering
During scattering a photon can be emitted.
Bremstrahlung
The particle’s velocity larger than the velocity of light in
the medium, ->
Cherenkov Radiation.
The particle crosses the boundary between two media -> Transition radiation.

37. Cherenkov radiation
Charge particles polarizes atoms along its track -> they become electric dipoles.
The time variation of the dipole field -> emission of electromagnetic radiation
v < c/n -> the dipoles are symmetric along the track
-> integration over all dipoles -> no field
v > c/n -> the dipoles are asymmetric along the track
-> cherenkov radiation
Cherenkov radiation contributes little to the energy loss .
For gases Z>7 less than 1% ionization
For He, H about 7% less than ionization

38. Cherenkov Light . c/n qc bc
Threshold: Cherenkov radiates on if b >bt = 1/n ;
Max is reached when b=1
Max is small for gases 1,40 in air and 450 in glass.
For a fix energy the threshold depends on the of the mass of the particle -> particle identification
The major problem of Cherenkov radiation is the modest light output:

39. Cherenkov light Threshold Cherenkov detectors
Only particles with velocity above the threshold are
detected.
Differential Cherenkov detectors
Only particles in a range of velocities are detected
Ring-Imaging Cherenkov detectors
Spherical mirrors are used to focus the cone of
Cherenkov light on a position sensitive light detector.
There are only ‘a few’ photons per event 
one needs highly sensitive photon detectors to measure the rings !

40. Delphi particle identification
Protons from L decay
p from K decay
K+- from D0 decay

41. Transition Radiation
Radiation emitted by ultra relativistic particles when they transverse the boarder of two materials of different dielectric permittivity.
The intensity of transition radiation is roughly proportional to the particle energy,
I = mg
-> Particle Identification at highly relativistic energies
The typical emission angle is 1/γ.

42. Transition Radiation Radiator Detector
The number of photon can be increased by placing many foils with low Z
The Photons would otherwise not be able to escape the radiator

43. Electromagnetic Interaction of Particles with Matter
The incoming
particle loses
energy .
The atoms are
excited or ionized.
The particle is deflected multiple scattering
During scattering a photon can be emitted.
Bremstrahlung
The particle’s velocity larger than the velocity of light in
the medium, ->
Cherenkov Radiation.
The particle crosses the boundary between two media -> Transition radiation.

44. Detecting Particles

45. Photons in Matter Photoelectric effect Compton Scattering
Pair Production

46. For E>>mec2=0.5MeV :  = 9/7X0
the attenuation length due to pair production is 9/7 times the radiation length.
σp.e = Atomic photoelectric effect
σRayleigh = Rayleigh (coherent) scattering–atom neither ionized nor excited
σCompton = Incoherent scattering (Compton scattering off an electron)
knuc = Pair production, nuclear field
ke = Pair production, electron field

47. Bremsstrahlung + Pair Production = Electromagnitic Shower

48. Detecting Particles

49. Strong Interaction
Hadrons interact with nuclei through the strong interaction and built up showers in matter.
The total cross section is given by
The average interaction length is given by
The interaction range can be calculated from the inelastic part of the hadronic cross section and one gets:
A = mass of the material (g/mol)
NA =Avogadros number (mol-1)
r = density (g/cm3)
rl gives the area density in g/cm2

50. Electromagnetic shower versus Hadronic shower
For the hadron interactions:
Many different final states
Up to 30% of incident energy may be lost
The cross section is a function of both energy and particle type

51. Particle identification
Time of Flight -> measure of velocity
Cherenkov radiation detectors
Transition radiation
Ionization
Development of showers separate electron and hadrons

52. Detecting Particles

53. Detecting Neutrino

54. If neutrinos have mass:
CNGS
If neutrinos have mass: ne nt nm
Muon neutrinos produced at CERN.
See if tau neutrinos arrive in Italy.

55. Neutrinos at CNGS: Some Numbers
For 1 day of CNGS operation, one expect:
protons on target 2 x 1017
pions / kaons at entrance to
decay tunnel x 1017
nm in direction of Gran Sasso
nm in 100 m2 at Gran Sasso 3 x 1012
nm events per day in OPERA  25 per day
nt events (from oscillation)  2 per year

56. Opera Experiment at Gran Sasso
Basic unit: brick
56 Pb sheets + 56 photographic films (emulsion sheets)
Lead plates: massive target
Emulsions: micrometric precision
8.3kg
brick
Brick Pb
Couche de gélatine photographique 40 mm n t
1 mm
10.2 x 12.7 x 7.5 cm3

57. Opera Experiment at Gran Sasso
31 target planes / supermodule
In total: bricks, 1766 tons
SM1
SM2 
Targets
Magnetic Spectrometers
First observation of CNGS beam neutrinos : August 18th, 2006
W. Riegler/CERN

58. Electromagnetic Interaction of Particles with Matter
The incoming
particle loses
energy .
The atoms are
excited or ionized.
The particle is deflected multiple scattering
During scattering a photon can be emitted.
Bremstrahlung
The particle’s velocity larger than the velocity of light in
the medium, ->
Cherenkov Radiation.
The particle crosses the boundary between two media -> Transition radiation.

59. Detectors Based on Excitation
Emission of photons of by excited
atoms, typically UV to visible light.
Scintillation Counters
Used for:
Triggers
Time of Flight …
1 cm thick
Inorganic for example NaI ns
Organic for example Plexiglass 3x ns

60. Scintillation Counters
Photons are being reflected towards the ends of the scintillator.
A light guide brings the photons to the Photomultipliers where the photons are converted to an electrical signal.

61. Rutherford’s experiment

62. Detectors based on Ionization
Gas detectors:
Wire Chambers
Drift Chambers
Time Projection Chambers
Transport of Electrons and Ions in Gases
Solid State Detectors
Transport of Electrons and Holes in Solids
Si- Detectors
Diamond Detectors
Detectors based on Ionization

63. Some General Features Resolution time Dead time 2ns 100 ns
ps ns
Spatial resolution
1 mm
2 mm – 2 mm
Drift chambers
Scintillators
Photo emulsion
Silicon pixel

64. High energy physics detectors
Tracking with momentum measurement
Electron and hadron separation
Particle identification
Energy determination
Triggering
Data acquisition

65. Two jet event

66. Why Four LEP Detectors?
DELPHI
OPAL L3
ALEPH