Particle Identification Techniques in High Energy Physics

Christian Joram CERN 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 K p

2. Time of Flight (TOF) p ? m 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. 16 May 2011 C. Joram Particle Identification Techniques 2 Idealistic views of an e+ e- elementary particle reaction

e  e  Z 0  qq (  hadronization) Z

q q time

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) !

16 May 2011 C. Joram Particle Identification Techniques 3 Certain measurements become only possible due to powerful PID: example B physics, the study of M2 = m 2 + m 2 + 2(E E  p p cosq ) hadrons containing the b quark 1 2 1 2 1 2 • B physics can shed light on the reason the “Invariant mass” Universe did not disappear soon after the Big Bang, from the annihilation of the matter and

antimatter: CP violation can give rise to an simulation excess of matter eg: B(B0  K+ p) > B(B0  K p+) LHCb • 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

 We need dedicated detectors and techniques to identify particles.

16 May 2011 C. Joram Particle Identification Techniques 4 Before we discuss dedicated particle ID methods and detectors …

… Tracking detectors, calorimeters and muon chambers implicitly provide also particle ID

16 May 2011 C. Joram Particle Identification Techniques 5 Innermost detector region: high precision silicon trackers (mm resolution) allow to identify primary (PV), secondary (SV) and tertiary vertices (TV)

! Several other tracks originating from PV are suppressed ! The path lengths l between the vertices tell us a lot LHCb about the lifetime of the p p particles : l = bc · gt

velocity lifetime (Lorentz boosted)

12 t (Bs) ~ 0.5 10 s  l = 0.5 mm b · g

12 t (Ds) ~ 1.5 10 s  l = 1.5 mm b · g

Scale in mm

16 May 2011 C. Joram Particle Identification Techniques 6 Different particle types behave differently in trackers and calorimeters

p, K, p

16 May 2011 C. Joram Particle Identification Techniques 7 Only muons can traverse meters of iron without creating a shower.

16 May 2011 C. Joram Particle Identification Techniques 8 Classical Particle ID techniques

• Specific Energy Loss dE/dx • Time of Flight (TOF) Mainly used for hadron (p, K, p) identification • Cherenkov Radiation p, K, p look (almost) the same in a tracker and • Transition Radiation calorimeter. However they have different rest masses ! 2 2 mp = 938 MeV/c , mK = 500 MeV/c , mp = 139 A very special effect. Works MeV/c2 practically only for electrons. 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.

16 May 2011 C. Joram Particle Identification Techniques 9 1. Specific Energy Loss dE/dx (See lecture by R. Venhof)

p  m0bgc Simultaneous measurement of p and dE/dx defines mass dE 1  ln (b 2g 2 ) m , hence the particle identity dx b 2 0

Average energy loss for e, m, p, K, p in 80/20 Ar/CH4 (NTP) (J.N. Marx, Physics today, Oct.78)

e p/K separation (2s) requires a dE/dx Dx m resolution of < 5% p

(arbitrary units) (arbitrary Not so easy to achive ! K • A real detector doesn’t measure but DE/Dx

relative p • Energy loss fluctuates and shows Landau tails (due to d-electrons). • dE/dx is very similar for minimum ionising particles (1-2 MeV·g-1·cm-2).

16 May 2011 C. Joram Particle Identification Techniques 10 Example: dE/dx in ALICE Time Projection Chamber L = 5m, 5m Ø (largest TPC ever built)

16 May 2011 C. Joram Particle Identification Techniques 11 … 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

16 May 2011 C. Joram Particle Identification Techniques 12 Event by event PID

Monte Carlo … or statistical analysis

e K p

16 May 2011 C. Joram Particle Identification Techniques 13 2. Time of Flight (TOF) L L L t  b  bc tc Combine TOF with momentum measurement

c2t2 p  m bg m  p 1 0 0 2 start stop L dm dp  dt dL  Mass resolution  g 2   m p  t L 

TOF for 1 m distance D TOF for 1 m distance L  1 1  1.E-07 1.E-08 Dt     e p - k   c  b1 b2  mu k - pi pi 1.E-09 Lc  (m2  m2 ) k mu - e 2 p2 1 2 p

1.E-08 1.E-10 TOF TOF (s)

TOF (s) TOF Time resolution s

D t required for p/K 1.E-11 separation at p= 1 GeV/c  300 ps  1.E-09 1.E-12 2 GeV/c 100 ps 0.1 1 10 0.1 1 10 10 GeV/c  4 ps 16 May 2011 p (GeV/c) C. Joram Particle Identification Techniquesp (GeV/c 14 Example: Measure TOF of particle produced in Heavy Ion collisions in ALICE

ALICE TOF (160.000 channels)

16 May 2011 C. Joram Particle Identification Techniques 15 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. 16 May 2011 C. Joram Particle Identification Techniques 16 Performance Test beam (2006)

Particle ID of a single HI collision

16 May 2011 C. Joram Particle Identification Techniques 17 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

16 May 2011 C. Joram Particle Identification Techniques 18 Propagating waves

• A stationary boat bobbing up and down on a lake, producing waves

19 • Now the boat starts to move, but slower than the waves • No coherent wavefront is formed

20 • Next the boat moves faster than the waves • A coherent wavefront is formed

21 • Finally the boat moves even faster • The angle of the coherent wavefront changes with the speed

cos q = vwave / vboat

22 … back to Cherenkov radiation

The dielectric medium is polarized by the passing particle.

“radiator” c A coherent wave front forms if v  c  particle medium n 1 n b  ( = refr. index) particle n vwave = c/n d·tanq

d vwave 1 qC cosqC   vparticle nb qC Cherenkov vparticle = bc with n  n() 1 cone radiator of limited thickness 1 Cherenkov 1 bthr   qC  0 qmax  arccos b n threshold n ‘saturated’ angle ( =1)

16 May 2011 C. Joram Particle Identification Techniques 23

Number of emitted photons per unit length and unit wavelength/energy interval

 /d 2 2 2

d N 2p z   1  2p z  dN  1   sin 2 q 2  2 2  2 C dxd   b n  

UV cut-off  2 2

d N 1 c hc d N E d

 with     const. /

dxd 2  E dxdE dN

2 d N 2  370/cm sin q DEdetector dxdE E 0 UV cut-off Cherenkov effect is a weak light source. There are only few photons produced.

dE Cherenkov dE Ionization 1keV/cm  0.001 dx dx

16 May 2011 C. Joram Particle Identification Techniques 24 Threshold Cherenkov detectors

 1  1  Exploit the  2 2  behavior of the Cherenkov light intensity  b n  Example: study of an Aerogel threshold detector for the BELLE experiment at KEK (Japan)

1.1 Goal: p/K separation 1.0 0.9 principle mirror b 0.8kaon n=1.03 radiator medium 0.7 particle 0.6 0.5 n=1.02 0.4

; light yield (a.u.) yield light ; 0.3

kaon 0.2 b b 0.1 n=1.01 PM 0.0 0123456

pkaon [GeV/c]

16 May 2011 C. Joram Particle Identification Techniques 25 Ring Imaging Cherenkov detectors

1  measure angle  intercept C-cone with a photosensitive plane Exploit cosq 

C nb  requires large area photon detectors, single photon sensitive

mirror Spherical

plane detection

qC particle

Short radiator Long gasous radiator Cherenkov angle in aerogel (n=1.02) Cherenkov angle in Neon gas (n=1.000067) 12 0.7

10 0.6 e e 0.5 8 mu 0.105 0.4 pi pi 6 k k 0.3 p p

4 Cherenkov angle angle Cherenkov(deg.) Cherenkov angle angle Cherenkov(deg.) 0.2 p/K difference 2 p/K difference at 10 GeV/c  6 mrad 0.1 at 50 GeV/c  5.5 mrad

0 0.0 0.1 1 10 100 0.1 1 10 100 1000 16 May 2011 p (GeV/c) C. Joram Particle Identification Techniques p (GeV/c) 26 Example: The LHCb RICH detectors

Two RICHes for p/K separation from 1 to ~100 GeV/c

LHCb

16 May 2011 C. Joram Particle Identification Techniques 27 LHCb RICH 2 In total (RICH 1+2) 484 HPDs

gas

particle 72mm 

16 May 2011 C. Joram Particle Identification Techniques 28 Stephanie Hansmann-Menzemer, LHCb status report, 102nd LHCC meeting.

K K efficiency

p K misidentification

16 May 2011 C. Joram Particle Identification Techniques 29 (there is an excellent review article 4. Transition radiation by B. Dolgoshein (NIM A 326 (1993) 434))

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. medium vacuum

A simple picture …

electron Correct relativistic theory by • Radiated energy per medium/vacuum boundary G. Garibian, Sov. Phys. JETP63 (1958) 1079 1 ± W   g W  g only high energetic e emit TR of 3 p detectable intensity  particle ID

2 Nee  plasma  1 finestructure   p     p  20eV (plastic radiators)      0me frequency  137  constant 

• Dipole radiation is Lorentz boosted  X-rays (keV) in very forward direction q 1 g 1 Typical photon energy:   4  pg e± ± 3 q  mrad e at E = 1 GeV; g ~ 2·10   10 keV 16 May 2011 C. Joram Particle Identification Techniques 30 W  Number of emitted photons per boundary N    is very small. ph   Need many transitions to produce a sizable signal. p

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)

R D R D R D R D alternating arrangement of radiators stacks and detectors  minimizes re-absorption

TR X-ray detectors:

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

• Mainly used: Gas detectors: MWPC, ) dE/dx TR (10 keV) Xe - - drift chamber, straw tubes… 200 e 500 e

Discrimination (1 cm cm (1 5 by threshold • Detector gas: sphoto effect  Z height Pulse  gas with high Z required, e.g. Xenon (Z=54) t • Intrinsic problem: detector “sees” TR and dE/dx 16 May 2011 C. Joram Particle Identification Techniques 31 Example: The ATLAS Transition Radiation Tracker (TRT)

The TRT is part of the ATLAS Inner Detector

Straw tube detectors (230.000) + polyepropylene foils / fibres 16 May 2011 C. Joram Particle Identification Techniques 32 4 mm Cross section view p e

electrons 30 mm

Gas mixture: 70% Xe + 27% CO2 + 3% O2

16 May 2011 C. Joram Particle Identification Techniques 33 Red dots = TRT e- hits

16 May 2011 C. Joram Particle Identification Techniques 34 Summary

There is a wide variety of techniques for identifying charged particles

• 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.

16 May 2011 C. Joram Particle Identification Techniques 35