Particle Identification in ALICE Detector

Zafar Yasin

PINSTECH (PAEC)

Outline

• Introduction to ALICE detector

• Introduction to ALICE Physics

• Determenation of time resolution

• Time resolution in ALICE TOF detector

• Particle Identification in ALICE detector

• The Large Hadron Collider (LHC) accelerates protons in a 27 km long tunnel.

• The LHC will also accelerate lead ions to make them collide at the highest energy ever.

• The acceleration process starts in LINAC 2 for protons and in LINAC 3 for lead ions.

• The protons accelerated in LINAC2, are injected to a Proton Synchrotron Booster with an energy of 50 MeV.

• In the Synchrotron, protons reach an energy of 1.4 GeV .• The SPS takes 26 GeV protons from the PS and brings them

to an energy of 450 GeV before extraction.• The LINAC 3 produces 4.2 MeV/u lead ions.

The Large Hadron Collider

The Large Hadron Collider• The Low Energy Injector Ring (LEIR) is used as a storage

and cooler unit.

• It provides ions to the Proton Synchrotron with an energy of 72 MeV/nucleon.

• Ions will be further accelerated by the Proton Synchrotron and the Super Proton Synchrotron before they are injected into the LHC where they reach an energy of 2.76 TeV/nucleon.

• The LHC consists of 1232 superconducting dipole magnets with double aperture that operate at up to 9 Tesla magnetic field.

• The accelerator also includes more than 500 quadrupole magnets and more than 4000 corrector magnets of many types.

Introduction to ALICE Physics

• ALICE is one of the big experiments at the LHC, which focuses on QCD, the strong-interaction sector of the Standard Model.

• It focuses on the study of heavy ion collisions at ultra-relativistic energies

• Its main goal is to observe a transition of ordinary matter into a plasma of quarks and gluons

• Nucleus–nucleus and proton–proton collisions will also be studied

• The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries.

Two major caregories of Detectors

Introduction to ALICE-Detector

• The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries.

• Its overall dimensions are 16 × 16 × 26 m 3 with a total weight of approximately 10 000 t.

• The experiment consists of 18 different detector systems each with its own specific technology

• The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum.

• This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei

LHC Experiments

ATLAS

CMS

Designed for high pTphysics in p-p

collisions

ALICE

Dedicated LHC HI experiment~ 9 km

CERN

LHC

The ALICE Detector

TRDTOFHMPID

PMD

PHOS TPC

Muon Arm

ITS

ALICE Set-up

Six Layers of silicon detectors for precision tracking in |η|< 0.9

3-D reconstruction (< 100mm) of the Primary Vertex

Tracking+Standalone reconstruction of very low momentum tracks

Particle identification via dE/dx for momenta < 1 GeV

SPD - Silicon Pixel

SDD - Silicon Drift

SSD - Silicon Strip

Secondary vertex Finding (Hyperons, D and B mesons)

Three tecnhnologies:

ALICE Inner Tracking System – ITS

• Can separate pions and protons up to 0.8 GeV/c

Efficient (>90%) tracking in η < 0.9

s(p)/p < 2.5% up to 10 GeV/c

Conventional TPC optimized for extreme track densities

ALICE Time Projection Chamber (TPC)

Two-track resolution < 10 MeV/c

PID with dE/dx resolution < 10%

Space-Point resolution 0.8 (1.2) mm in xy,(z), occupancy from 40% to 15%

Tranisition Radiation Detector

Surrounds ALICE TPC• radial position 2.9 < r < 3.7 m• maximal length 7 m• full azimuthal coverage• |η| < 0.9• 540 detector modules arranged in:•ϕ: 18 super modules• r : 6 layers• z: 5 stacks• 694 m2 active area• 28 m3 detector gas of Xe/CO2• 30 tons• 10 M Euro and 250 person years

ALICE Time Of Flight – TOF

Large array at R ~ 3.7 m, covering | η| < 0.9 and full φ

Extensive R&D, from TB data:

Intrinsic Resolution ~ 40 psEfficiency > 99%Can separate π/K and K/p up to 2.5 GeV/c

Readout pads3.5x2.5 cm2

122 c

m

TOF basic element:double-stack Multigap RPC strip

Occupancy < 15% (O(105) readout channels)

2x5 gas gapsof 250mm

(fully installed & commissioned)ITS, TPC, TOF, HMPIDMUON, PMD, V0, T0FMD, ZDC, ACORDEPartially completed:● TRD (7/18)● PHOS (3/5)● EMCAL (4/11)ALICE capabilities for first physics:• 100% for hadron physics• partial for electron and photon physics

Current Status of the ALICE-Detector

Tracking Efficiency / Fraction of Fake Tracks for dN/dy = 2000, 4000, 6000, 8000

For dN/dy = 2000 ÷ 4000,efficiency > 90%,fake track probability < 5%!!!

For dN/dy = 2000 ÷ 4000,efficiency > 90%,fake track probability < 5%!!!

Full chain, ITS + TPC + TRD

ALICE Tracking Performance

ALICE PID

efficiency

contamination

p dependence of:

Central Pb + Pb HIJING events – kaon case

ITSstand-alone TPC

stand-alone

TOFstand-alone

Combining the PID information from different detectors allows a weaker momentum dependence of the efficiency (contamination) which stays higher (lower) or at least equal

than with stand-alone detectors!!!

Combining the PID information from different detectors allows a weaker momentum dependence of the efficiency (contamination) which stays higher (lower) or at least equal

than with stand-alone detectors!!!

ITS & TPC & TOFcombined!!combined!!!!

• TOF detector in ALICE is dedicated to charged particle identification over a large part of the phase space

• The basic Physics of ALICE detector demands a TOF detector with outstanding characteristics

• The detector should cover the momentum range from 0.5 GeV/c to 2.5 GeV/c

• The intrinsic time resolution must be well below 100 ps and over all time resolution 160 ps

• Since a large area has to be covered (~160 sq. meter), a gaseous detector is the only choice

• Multiplate Resistive Parallel Plate chambers are selected

TOF detector in ALICE

• Electric field should be high and uniform across the plates• Ionisation produced by a through-going particle will start

an avalanche

Basic idea of time of flight

• This method is used to obtain energy spectrum to identify the particles.

• It relies on accurate and fast time measurements• If time of flight (tof) is known for a particle that traverses a

given flight path (tp), the energy of the particle Ep, is given by

• Ep = E0 (1/(1-β2)1/2 -1)where,β = fp/tof. 1/c, c is the velocity of light.E0 = Particle rest massTo measure TOF, there is need a pulse to start a TDC (time

to digitical convertor) and a second pulse to stop the TDC. Both pulses are provided by the detectors.

• The energy resolution will be a function of the timing characteristics of the electronics and of the flight path.

• The longer the flight path the better the resolution.

Advantages of MRPC• It operates at atmospheric pressure; • The construction requires commercially available glass; • The signal is the analogue sum of signals from many gaps,

so there is no late tail and the charge spectrum is not of an exponential shape, it has a peak well separated from zero;

• The resistive plates quench the streamers so there are no sparks, thus high gain operation becomes possible;

• Both an array of single cells and a multicell strip design produce good uniformity: for the second option, the geometric arrangement is far simpler and it explains the choice of this design.

d

.Primaryelectron

N ~ exp (α . d)

Cathode

Anode

2)()exp(

dd

CemV

αα

=

N (d) = N0 exp (α. d)

Priciple and Operation of MRPCThe number of secondary electrons are

given by:

N0 = number of primary electrons

α = first Towsend coefficient

d = drift path

The voltage pulse generated by a fragment:

m = total number of primary electrons

C = the detector capacitance

Determination of Time Resolution

Experimental setup for determination of time resolution

Schematic overview of the electronic setup used to calculate time resolution.

CFD

ADC2

PPAD2

PPAD1

Attenuator

Pre-amplifier

Cf252

CFD

OR/AND ADC1

LATCH

OR

TDC

Scaler2 Scaler1

Gate generator

Fan out

VETO

Four fold logic unit

Computer

Stop

Start

Stop2 Stop1

Start

NIM

ADC

Gate

Fanout

Interrupt

Delay

Electronic Setup

400 440 480 520 560 600 640 680 720 760 800

500100015002000250030003500400045005000550060006500

Cou

nts/s

Voltage (V)

Pressure = 10 Torr

520 560 600 640 680 720 760 800

500100015002000250030003500400045005000550060006500

Cou

nts/s

Voltage (V)

Pressure = 15 Torr

520 560 600 640 680 720 760 800

500100015002000250030003500400045005000550060006500

Cou

nts/s

Voltage (V)

Pressure = 18 Torr

Count rate as a function of voltage and pressure using a PPAD for a

252Cf .

Testing of Detectors

14 15 16 17 18 19 20100

150

200

250

300

350

400

450

500

Cou

nts/s

P ressu re (T orr)

V oltage = 570 V

Effect of pressure on count rate at constant voltage.

Offline analysis is performed using the computer code ROOT.

The ROOT software is an object-oriented (C++) data analysis package developed at CERN.

It was developed keeping in mind the high energy physics experiments and several laboratories around the world use ROOT for the analysis of data.

Data analysis

Data analysis

A pulse height spectrum from a PPAD A time spectrum

A time spectrumA time spectrum

Data analysis

A coincidence time spectrum A time spectrum

A pulse height spectrum A pulse height spectrum

If the intrinsic time resolutions of the start and stop detectors are equal, then we can write,

Δt2exp = Δt2

kin + Δt2geom + 2 Δt2

intr

In the present geometry the value of the Δtkin comes out to be 0.6 ns and Δtexp 1.25 ns. If we assume the Δtgeom to be 0.17 ns, then the intrinsic time resolution is 0.76 ns.

Time resolution

400 420 440 460 480 500100

200

300

400

500

600

700

FWH

M (p

s)

Voltage (V)

0 2 4 6 8 10

0

2

4

6

8

10

8 12 16 20 24

1.05

1.20

1.35

1.50

1.65

1.80

1.95

Tim

e re

solu

tion

(FW

HM

)

Pressure (Torr)

0 5 10 15 20 250.25

0.50

0.75

1.00

1.25

1.50

1.75

Effe

ctiv

e tim

e re

solu

tion

(ns)

Pressure (torr)

0 20 40 60 80

100

200

300

400

5000 20 40 60 80

100

200

300

400

500

5.5 MeV alpha particles 8.1 MeV protons 2.3 MeV protonsTi

me

reso

lutio

n(FW

HM

)

Gas pressure(torr)

Effect of voltage and presssure and pressure on time resolution

Time Resolution

TOF time resolution as a function of applied voltage

Time Resolution

Efficiency and time resolution versus high voltage

Random coincidences

True coincidences

Time

Counts

Data analysis

A schematic TDC spectrum

Focal plane sum spectrum

Focal plane sum spectrum after ADC and TDC cut

Data analysis

Particle identification capabilitiesof the detectors

ALICE TOF Particle Identification

Reconstructed mass with the TOF detector in the momentum range 0.5 GeVfc<p<2.5 GeV/c for 50 HIJING Pb-Pb events at B=0.4 T.Individual mass distributions of the true π, K, p are also shown as hatched area.

ALICE TOF Particle Identification

Mass separation as a function of momentum with the Time of Flight detector in Pb-Pb collision

Likelihood can be based on:• total deposited charge (LQ)• deposited charge/position (LQX)• Exceed design goal of factor 100 pion rejection for isolated tracks

Test beam measurement at CERN PS with electron and pion beam

Electron Identification in TRD

ALICE is Ready for Data

`̀Take some more tea,Take some more tea,’’ the March Hare said to Alice, very the March Hare said to Alice, very earnestly. earnestly. `I've had nothing yet,' Alice replied in an offended tone, `I've had nothing yet,' Alice replied in an offended tone, `so I can't take more.`so I can't take more.’’`You mean you can't take LESS,' said the Hatter: `it's very `You mean you can't take LESS,' said the Hatter: `it's very easy to take MORE than nothing.easy to take MORE than nothing.’’

L. Carroll, Alice in WonderlandL. Carroll, Alice in Wonderland