06.JORAM Particle ID.pdf

Particle Identification Techniques in High Energy Physics

Christian JoramCERN

http://www.epn-campus.eu/esi-2011/overview-and-objectives/

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.


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-


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”


Before we discuss dedicated particle ID methods and detectors …

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


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 !

http://lhcb-public.web.cern.ch/lhcb-public/Images_2010/BsDsMuNu.png


Different particle types behave differently in trackers and calorimeters

p, K, p

n


Only muons can traverse meters of iron without creating a shower.


• 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


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


Example: dE/dx in ALICE Time Projection Chamber L = 5m, 5m Ø (largest TPC ever built)


… 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


Event by event PID

… or statistical analysisMonte Carlo

p

K

e

p


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


ALICE TOF (160.000 channels)

Example: Measure TOF of particle produced in Heavy Ion collisions in ALICE


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.


PerformanceTest beam (2006)

Particle IDof a single HI collision


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


… 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


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


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


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


LHCb

Example: The LHCb RICH detectors

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


particle

gas

LHCbRICH 2 In total (RICH 1+2)

484 HPDs

72mm


K K efficiency

p K misidentification

Stephanie Hansmann-Menzemer, LHCb status report, 102nd LHCC meeting.


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:


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


Example: The ATLAS Transition Radiation Tracker (TRT)


electrons

Cross section view

p

e

4 mm

30 mm

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


Red dots = TRT e- hits


• 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

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06.JORAM Particle ID.pdf