CERN Summer Student Lectures 2003

1
Particle Identification

• dE/dx measurement
• Time of flight
• Cherenkov detectors
• Transition radiation detectors

Particle Identification
p
p
K
K
p
p
m
m
2
DELPHI
Why particle ID ?
A charmless B decay
1 K 2 p in final state
Who is who ?
3
Specific energy loss
Particle ID using the specific energy loss dE/dx
Simultaneous measurement of p and dE/dx defines
mass m, hence the particle identity
e
m
m
m
p/K separation (2s) requires a dE/dx resolution
of lt 5
p
p
p
K
K
K
Average energy loss for e,m,p,K,p in 80/20 Ar/CH4
(NTP) (J.N. Marx, Physics today, Oct.78)
p
p
p

But Large fluctuations Landau tails !
4
Specific energy loss (backup)
Improve dE/dx resolution and fight Landau tails

• Chose gas with high specific ionization
• Devide detector length L in N gaps of thickness
  T.
• Sample dE/dx N times

(B. Adeva et al., NIM A 290 (1990) 115)
4 wires
1 wire
L most likely energy loss A average energy loss
(M. Aderholz, NIM A 118 (1974), 419)
Dont cut the track into too many slices ! There
is an optimum for each total detector length L.

• calculate truncated mean, i.e. ignore samples
  with (e.g. 40) highest values
• Also pressure increase can improve resolution,
  but reduced rel. rise due to density effect !

5
Specific energy loss
Example ALPEPH TPC Gas Ar/CH4 90/10 Nsamples
338, wire spacing 4 mm dE/dx resolution 4.5
for Bhabhas, 5 for m.i.p.s
log scale !
linear scale !
6
Specific energy loss
dE/dx can also be used in Silicon
detectors Example DELPHI microvertex detector
(3 x 300 mm Silicon)
DE (a.u.)
log p GeV/c
DE (a.u.)
log p GeV/c
7
Time of flight
Particle ID using Time Of Flight (TOF)
start
stop
Combine TOF with momentum measurement
Mass resolution
TOF difference of 2 particles at a given momentum
Dt for L 1 m path length
st 300 ps p/K separation up to 1 GeV/c
8
Time of flight
Example CERN NA49 Heavy Ion experiment
detail of the grid
Small, but thick scint. 8 x 3.3 x 2.3 cm
Long scint. (48 or 130 cm), read out on both
sides
TOF requires fast detectors (plastic
scintillator, gaseous detectors), approporiate
signal processing (constant fraction
discrimination, corrections continuous
stability monitoring.
9
Time of flight
From g conversion in scintillators
System resolution of the tile stack
L 15 m
Trel. T / Tp
NA49 combined particle ID TOF dE/dx (TPC)
10
Interaction of charged particles
Remember energy loss due to ionisation There are
other ways of energy loss !

• A photon in a medium has to follow the dispersion
  relation

schematically !

• For soft collisions energy and momentum
  conservation ? real photons

? Emission of Cherenkov photons if
11
Cherenkov detectors

• Cherenkov radiation
• Cherenkov radiation is emitted when a charged
  particle passes a dielectric medium with velocity
  

threshold
saturated angle (b1)
Number of emitted photons per unit length and
unit wavelength interval
12
Cherenkov detectors

• Energy loss by Cherenkov radiation small compared
  to ionization (?1)

Number of detected photo electrons
DE E2 - E1 is the width of the sensitive window
of the photodetector (photomultiplier,
photosensitive gas detector...)
Example for a detector with and
a Cherenkov angle of one expects
photo electrons
13
Cherenkov detectors

• Particle ID with Cherenkov detectors
• 
  

• Detectors can exploit ...
• Nph(b) threshold detector
• q(b) differential and
• Ring Imaging Cherenkov detectors
  RICH

(do not measure qC)
Threshold Cherenkov detectors
principle
Example study of an Aerogel threshold detector
for the BELLE experiment at KEK (Japan) Goal
p/K separation
bkaon
14
Cherenkov detectors

• Ring Imaging Cherenkov detectors (RICH)
• RICH detectors determine qC by
• intersecting the Cherenkov cone
• with a photosensitive plane
• ? requires large area photosensitive detectors,
  e.g.
• wire chambers with photosensitive detector gas
• PMT arrays

(J. Seguinot, T. Ypsilantis, NIM 142 (1977) 377)
.
.
.
.
.
.
.
.
.
.
.
n 1.28 C6F14 liquid
DELPHI
p/K
p/K/p
K/p
n 1.0018 C5F12 gas
p/h
p/K/p
K/p
? minimize sq ? maximize Np.e.
Detect N photons (p.e.) ?
15
Cherenkov detectors
Principle of operation of a RICH detectors
DELPHI RICH
2 radiators 1 photodetector
A RICH with two radiators to cover a large
momentum range. p/K/p separation 0.7 - 45
GeV/c DELPHI and SLD
spherical mirror
C5F12 (40 cm, gas) C4F10 (50 cm, gas)
Photodetector TMAE-based
C6F14 (1 cm, liquid)
(W. Adam et al. NIM A 371 (1996) 240)
Two particles from a hadronic jet (Z-decay) in
the DELPHI gas and liquid radiator hypothesis
for p and K
16
Cherenkov detectors
The mirror cage of the DELPHI Barrel RICH (288
parabolic mirrors)
17
Cherenkov detectors
Marriage of mirror cage and central detector
part of the DELPHI Barrel RICH.
18
Cherenkov detectors

• Performance of DELPHI RICH (barrel) in hadronic Z
  decays

Liquid radiator
gas radiator
p
p
p
K
p
K
(E. Schyns, PhD thesis, Wuppertal University 1997)
19
Transition radiation detectors

• Transition radiation detectors

(there is an excellent review article by B.
Dolgoshein (NIM A 326 (1993) 434))
TR predicted by Ginzburg and Franck in 1946
Electromagnetic radiation is emitted when a
charged particle traverses a medium with a
discontinuous refractive index, e.g. the
boundaries between vacuum and a dielectric layer.
medium
vacuum
A (too) simple picture
electron
A correct relativistic treatment shows that
(G. Garibian, Sov. Phys. JETP63 (1958) 1079)
? Radiated energy per medium/vacuum boundary
20
Transition radiation detectors
? Number of emitted photons / boundary is small
Need many transitions ? build a stack of many
thin foils with gas gaps
? X-rays are emitted with a sharp maximum at
small angle ? TR stay close to track
? Emission spectrum of TR Typical energy
? photons in the keV range

• Simulated emission spectrum of a CH2 foil stack

21
Transition radiation detectors

• TR Radiators
• stacks of CH2 foils are used
• hydrocarbon foam and fiber materials
• Low Z material preferred to keep re-absorption
  small (?Z5)

R D R D R D R D
sandwich of radiator stacks and detectors ?
minimize re-absorption
TR X-ray detectors

• Detector should be sensitive for 3 ? Eg ? 30 keV.

• Mainly used Gaseous detectors MWPC, drift
  chamber, straw tubes
• Detector gas sphoto effect ? Z5
• ? gas with high Z required,
• e.g. Xenon (Z54)

Intrinsic problem detector sees TR and dE/dx
dE/dx ?200 e-
TR (10 keV) ?500 e-
Pulse height (1 cm Xe)
Discrimination by threshold
t
22
ATLAS Transition Radiation Tracker
A prototype endcap wheel. X-ray
detectorstraw tubes (4mm) (in total ca. 400.000
!) Xe based gas
TRT protoype performance
Pion fake rate at 90 electron detection
efficiency p90 1.58
23
Particle Identification

• Summary
• A number of powerful methods are available to
  identify particles over a large momentum range.
• Depending on the available space and the
  environment, the identification power can vary
  significantly.
• A very coarse plot .

e identification
p/K separation
24
Detector Systems
Lets find some tools and put everything
together !
25
Detector Systems
Detector Systems

• Remember we want to have info on...
• number of particles
• event topology
• momentum / energy
• particle identity

Cant be achieved with a single detector !
? integrate detectors to detector systems
Geometrical concepts Fix target geometry
Collider Geometry Magnet spectrometer
4p Multi purpose detector
traget tracking muon filter
N
S
barrel endcap endcap
beam magnet calorimeter
(dipole)

• Limited solid angle dW coverage
• rel. easy access (cables,
• maintenance)

• full dW coverage
• very restricted access

26
Detector Systems
collider geometry cont.
Magnetic field configurations
solenoid
toroid
B
B
Imagnet
coil
Imagnet

• Large homogenous field
• inside coil
• - weak opposite field in return yoke
• - Size limited (cost)
• - rel. high material budget
• Examples
• DELPHI (SC, 1.2T)
• L3 (NC, 0.5T)
• CMS (SC, 4T)

• Rel. large fields over large volume
• Rel. low material budget
• - non-uniform field
• - complex structure
• Example
• ATLAS (Barrel air
• toroid, SC, 0.6T)

27
Detector Systems
Typical arrangement of subdetectors
Low density ? high density high precision
? low precision high granularity ? low
granularity track density ? 1/r2
m
e-
g
p
vertex location (Si detectors) ?
main tracking (gas or Si detectors) ?
particle identification ?
e.m. calorimetry ?
magnet coil ?

hadron calorimetry / return yoke ?
muon identification / tracking ?
ATLAS and CMS require high precision tracking
also for high energetic muons ? large muon
systems with high spatial resolution behind
calorimeters.
28
Detector Systems
Some practical considerations before building a
detector

• Find compromises and clever solutions
• Mechanical stability, precision ? distortion of
  resolution (due multiple scattering, conversion
  of gammas)
• Hermeticity ? routing of cables and pipes
• Hermeticity ? thermal stability
• Hermeticity ? accessibility, maintainability
• Compatibility with radiation
• and always keep an eye on cost

Composites are very interesting candidates, e.g.
glass or carbon fiber reinforced epoxy materials.
29
Detector Systems
Radiation damage to materials
Radiation levels in CMS Inner Tracker (0 lt z lt
280 cm)
(J/Kg)
no damage
moderate damage
destruction
H. Schönbacher, M. Tavlet, CERN 94-07
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Detector Systems
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Detector Systems
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Detector Systems
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Detector Systems