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Particle Identification Techniques in High Energy Physics
Christian JoramCERN
16 May 2011 C. Joram Particle Identification Techniques 2
p
K
m
p
?
Introduction
• Why Particle Identification (PID) ?
• Very briefly: “Implicit” PID calorimetry, muon detection, secondary vertex
Classical Particle ID techniques: principles, limitations, examples
1. Specific Energy Loss dE/dx
2. Time of Flight (TOF)
3. Cherenkov Radiation
4. Transition Radiation Many thanks to Crispin Williams, Roger Forty, Christoph Rembser (all CERN), Alexander Kalweit (GSI) for material used in this lecture.
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e-e+
Z
Idealistic views of an elementary particle reaction
Usually we can not ‘see’ the reaction itself, but only the end products of the reaction.
In order to reconstruct the reaction mechanism and the properties of the involved particles (e.g. Z-boson, Higgs boson), we want the maximum information about the end products: charge, momentum, identity (=mass) !
ion)hadronizat (
0
qqZee
time
q q-
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Certain measurements become only possible due to powerful PID: example B physics, the study of hadrons containing the b quark
• B physics can shed light on the reason the Universe did not disappear soon after the Big Bang, from the annihilation of the matter and antimatter: CP violation can give rise to an excess of matter
eg: B(B0 K+ p) > B(B0 K p+)
• In a tracking detector, p, K, p will just look the same !
• If one makes combinations of all two-body B decays many different modes overlap→ very difficult to study their properties
• Applying particle ID (p, K, p), the different components can be separately studied
LHC
bsi
mu
lati
on
We need dedicated detectors and techniques to identify particles.
M2 = m12 + m2
2 + 2(E1E2 p1p2 cosq )
“Invariant mass”
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Before we discuss dedicated particle ID methods and detectors …
… Tracking detectors, calorimeters and muon chambers implicitly provide also particle ID
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Innermost detector region: high precision silicon trackers (mm resolution) allow to identify primary (PV), secondary (SV) and tertiary vertices (TV)
LHCb
Scale in mm
The path lengths l betweenthe vertices tell us a lot about the lifetime of the particles :
l = bc · gt
velocity lifetime (Lorentz boosted)
t (Bs) ~ 0.5 1012 s
l = 0.5 mm b · g
t (Ds) ~ 1.5 1012 s
l = 1.5 mm b · g
pp
! Several other tracks originating from PV are suppressed !
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Different particle types behave differently in trackers and calorimeters
p, K, p
n
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Only muons can traverse meters of iron without creating a shower.
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• Specific Energy Loss dE/dx
• Time of Flight (TOF)• Cherenkov Radiation
• Transition Radiation
Mainly used for hadron (p, K, p) identification
p, K, p look (almost) the same in a tracker and calorimeter. However they have different rest masses !mp = 938 MeV/c2, mK = 500 MeV/c2, mp = 139 MeV/c2
A tracker in magnetic field measures their momentum p. If we are able to measure also their velocity v = bc, we can derive their rest mass m0= p/bcg and hence their identity.
dE/dx, TOF and Cherenkov measure the velocity of a particle.
A very special effect. Works practically only for electrons.
Classical Particle ID techniques
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1. Specific Energy Loss dE/dx (See lecture by R. Venhof)
( )22
2ln
1gb
b
dx
dE
cmp bg0Simultaneous measurement of p and dE/dx defines mass m0, hence the particle identity
Average energy loss for e, m, p, K, p in 80/20 Ar/CH4 (NTP)(J.N. Marx, Physics today, Oct.78)
p/K separation (2s) requires a dE/dxresolution of < 5%
e
Not so easy to achive !
• A real detector doesn’t measure<dE/dx> but DE/Dx
• Energy loss fluctuates and showsLandau tails (due to d-electrons).
• dE/dx is very similar for minimumionising particles (1-2 MeV·g-1·cm-2).
p
K
p
m
p
K
p
m
p
K
p
m
(arb
itra
ry u
nit
s)re
lati
ve
Dx
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Example: dE/dx in ALICE Time Projection Chamber L = 5m, 5m Ø (largest TPC ever built)
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… the result of lots of• careful calibration, e.g. channel-by-channel gain equalization, pressure, temperature, • and data treatment (truncated mean to suppress d-electrons)
dE/dx resolution
~4.5% for 160 clusters
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Event by event PID
… or statistical analysisMonte Carlo
p
K
e
p
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2. Time of Flight (TOF)
c
Lt
b
12
22
0 L
tcpm
start stop
Combine TOF with momentum measurement
L
dL
t
dt
p
dp
m
dm 2gMass resolution
bg0mp
Ltc
Lb
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
0.1 1 10
DTO
F (s
)
p (GeV/c
D TOF for 1 m distance
p - k
k - pi
mu - e
1.E-09
1.E-08
1.E-07
0.1 1 10
TOF
(s)
p (GeV/c)
TOF for 1 m distance
e
mu
pi
k
p
( )2
2
2
12
21
2
11
mmp
Lc
c
Lt
D
bb
Time resolution st
required for p/K separation at p=1 GeV/c 300 ps2 GeV/c 100 ps10 GeV/c 4 ps
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ALICE TOF (160.000 channels)
Example: Measure TOF of particle produced in Heavy Ion collisions in ALICE
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This is real data! Tracks from a single HI collision. To measure TOF of all these particles, the detector must be finely segmented.
Principle of the ALICE Multi Gap Resistive Plate chamber.
Based on 12 cheap glass plates and 10 gas gaps (two stacks of 5 gas gaps) each gap is 250 micron wide.
Built in the form of strips, each with an active area of 120 x 7.2 cm2, readout by 96 pads 160.000 channels.
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PerformanceTest beam (2006)
Particle IDof a single HI collision
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3. Cherenkov Radiation
A charged particle, moving through a medium at a speed which is greater than the speed of light in the medium, produces Cherenkov light.
Classical analogue: fast boat on water
19
Propagating waves
• A stationary boat bobbing up and down on a lake, producing waves
20
• Now the boat starts to move, but slower than the waves
• No coherent wavefront is formed
21
• Next the boat moves faster than the waves
• A coherent wavefront is formed
22
• Finally the boat moves even faster
• The angle of the coherent wavefront changes with the speed
cos q = vwave / vboat
q
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… back to Cherenkov radiation
1)(with
1cos
particle
wave
bq
nn
nv
vC
01
Cthrn
qb Cherenkovthreshold n
1arccosmax q ‘saturated’ angle (b=1)
qC
d
d·tanq
The dielectric medium is polarized by the passing particle.
A coherent wave front forms if n
ccv mediumparticle
(n = refr. index)
n
1particle b
“radiator”
qC
vwave = c/n
vparticle = bc
radiator of limited thickness
Cherenkovcone
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Number of emitted photons per unit length and unit wavelength/energy interval
C
z
n
z
dxd
Ndq
p
b
p
2
2
2
222
22
sin21
12
dN
/d
detector
22
sin /cm370 EdxdE
NdD q
dN
/dE
E0
.with 1 2
2
2
constdxdE
Nd
E
hcc
dxd
Nd
UV cut-off
Cherenkov effect is a weak light source. There are only few photons produced. UV cut-off
IonizationCherenkov
001.0keV/cm1dx
dE
dx
dE
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Example: study of an Aerogelthreshold detector for the BELLE experiment at KEK (Japan)
Goal: p/K separation
Threshold Cherenkov detectors
0123456
pkaon [GeV/c]
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
b kaon ; light yield (a.u.)
n=1.03
n=1.02
n=1.01
bkaon
principle
particle
mirror
radiator medium
PM
Exploit the behavior of the Cherenkov light intensity
22
11
nb
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Ring Imaging Cherenkov detectors
bq
nC
1cos Exploit
qC
measure angle intercept C-cone with a photosensitive plane requires large area photon detectors, single photon sensitive
0
2
4
6
8
10
12
0.1 1 10 100
Ch
ere
nko
v an
gle
(d
eg.
)
p (GeV/c)
Cherenkov angle in aerogel (n=1.02)
e
mu
pi
k
p
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.1 1 10 100 1000
Ch
ere
nko
v an
gle
(d
eg.
)
p (GeV/c)
Cherenkov angle in Neon gas (n=1.000067)
e
0.105
pi
k
p
p/K differenceat 10 GeV/c 6 mrad
p/K differenceat 50 GeV/c 5.5 mrad
Sph
erical m
irror
particle
detectio
n
plan
e
Shortradiator Long gasous
radiator
detectio
n
plan
e
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LHCb
Example: The LHCb RICH detectors
Two RICHes for p/K separation from 1 to ~100 GeV/c
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particle
gas
LHCbRICH 2 In total (RICH 1+2)
484 HPDs
72mm
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K K efficiency
p K misidentification
Stephanie Hansmann-Menzemer, LHCb status report, 102nd LHCC meeting.
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4. Transition radiation
constant
urefinestruct
137
1 radiators) (plastic eV20
frequency
plasma
3
1
0
2
gg
p
e
ep
p
m
eN
WW
only high energetic e± emit TR of detectable intensity particle ID
medium vacuum
Transition Radiation was predicted by Ginzburg and Franck in 1946.TR is electromagnetic radiation emitted when a charged particle traverses a medium with a discontinuous refractive index, e.g. the boundaries between vacuum and a dielectric layer. The temporary polarization of the medium leads to a dipole varying in time radiation.
A simple picture …
• Radiated energy per medium/vacuum boundary
(there is an excellent review article by B. Dolgoshein (NIM A 326 (1993) 434))
Correct relativistic theory by G. Garibian, Sov. Phys.
JETP63 (1958) 1079
electron
• Dipole radiation is Lorentz boosted X-rays (keV) in very forward direction
gq 1
e±
mradq
g p41
e± at E = 1 GeV; g ~ 2·103 keV10
Typical photon energy:
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Number of emitted photons per boundary is very small. Need many transitions to produce a sizable signal.
p
ph
WN
R D R D R D R Dalternating arrangement of radiators stacks and detectors
minimizes re-absorption
TR Radiators:
• stacks of thin foils made out of CH2 (polyethylene), C5H4O2 (Mylar)
• hydrocarbon foam and fiber materials. Low Z material preferred to keep re-absorption small (Z5)
• Detector should be sensitive for 3 Eg 30 keV.
• Mainly used: Gas detectors: MWPC, drift chamber, straw tubes…
• Detector gas: sphoto effect Z5
gas with high Z required, e.g. Xenon (Z=54)
TR X-ray detectors:
• Intrinsic problem: detector “sees” TR and dE/dx
Pu
lse
hei
ght
(1 c
m X
e)t
dE/dx
200 e-
TR (10 keV)
500 e-
Discrimination
by threshold
Straw tube detectors (230.000) + polyepropylene foils / fibres
The TRT is part of the
ATLAS Inner Detector
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Example: The ATLAS Transition Radiation Tracker (TRT)
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electrons
Cross section view
p
e
4 mm
30 mm
Gas mixture: 70% Xe + 27% CO2 + 3% O2
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Red dots = TRT e- hits
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• Ionization energy loss dE/dx is provided “for free” by existing tracking detectors but usually gives limited separation, at low p
• Time Of Flight provides excellent performance at low momentum. Ultrafast photon detectors [~O(10ps)] and radiators extend the range quite a bit.
• Cherenkov detectors can cover a large p-range, depending on type and radiator
• Transition radiation, usually implemented in a tracking detector, is useful for electron identification.
There is a wide variety of techniques for identifying charged particles
Summary