Combined particle identification

with the ITS and TPC B.Batyunya, S.Zaporozhets, Dubna, JINR

04.09.03, ALICE Offline Week

1. PID in the ITS and TPC separately.1.2 PID procedure and software structure.1.3 PID efficiency and contamination for 480 HIJING events.1.4 The fake tracks problem.

2. Combined PID with the ITS and TPC.

3. Conclusions.

-- The cut functions are used for the PID in the good separation region.-- The probability weights by two-three gaussians(the special one for the tail approximation) are calculated in the overlapping regions.

The cut functions and the gaussian parameters are stored in the AliITSPid(AliTPCPid) class and the output information is written to the TreeR in theAliITStracksV2Pid.root (AliTPCtracksPid.root), one Tree for one event.

Signal distributios (mip) for 480 HIJNG events at the reconstructedmomentum p= 0.3-0.4 GeV/c.

--The contamination in the signal region 1-2 mips is a consequence of the ‘fake’tracks which ones contain one or more points from the other tracks (mostly from the pion ones) !

--The tail problem is seen !

-- the relative signal resolutions(sigma/mean) are (10-12) % for all particle kinds

PID in the TPC

480 HIJING events

Distributions of the truncated mean (half out of 140) TPC signals(mips)in the momentum region of 0.35-0.40 GeV/c.

!! The tail problem is seen.

The signal resolutions are (6.5-8)% for pions, (5.5-7)% for kaons,

(6-8)% for protons depends from the momentum.

Momentum dependences ofthe signal resolutions for the ITS

Momentum dependences ofthe signal resolutions for the TPC

Correlation plot of the SDD/SSD signals (mip) vs momentum afterthe tracking and the particle identification for 100 HIJING events.

The same but for explicit (generated) momentum.

PID efficiency - the ratio of the correctly identified particle numbers to the ones entered to the PID procedure.

480 HIJING

events

PID contamination - the ratio of the misidentified particle numbers to the all

identified ones.

!! One can see the problem of the efficiency decrease for kaons and protons in low momentum region as a consequence of the ‘fake’ tracks.

480 HIJNG events

The ITS PID efficiencies and contamination with the ‘fake’tracks removing.

!! No down of the efficiencies is seen in the low momentum region.

TPC

480 HIJING events

0.2T

Correlation plot of the TPC signals(mips) vs. particlemomentum at 0.2T magnetic field.

The electron selection is possible at p = (0.2-04) GeV/c

TPC0.4T 480 HIJNG

events

Correlation plot of the TPC signals(mips) vs. momentumat 0.4T magnetic field.

Momentum GeV/c

p

k

e

!! The problem for the electron selection at p = (0.2-0.4) GeV/c because the worse signal resolution.

480 HIJING events

The TPC PID efficiencies and contaminationsfor different particle kinds.

480 HIJING events

Correlation of the ITS and TPC signals in momentumregion 370-380 MeV/c.

TPC TPC

Signals for HIJING events. Signal probability densities.

480 HIJING events

The PID efficiensies and contaminations for TPC(black points)and for ITS+TPC(colour points).

The improvement of the combined (ITS+TPC) efficiencies isseen as compared with the TPC ones at p > 0.5 GeV/c.The fake track broblem is kept at p < 0.3 GeV/c.

Conclusions.

:The combined PID with the ITS+TPC increases the PIDefficiencies and decreases the contaminations for pions,kaons and protons at p > 0.5 GeV/c.

The fake track problem is kept at p < 0.3 GeV/c.