Particle Identification @ STAR

[email protected] Pylos June 2002 1

Particle Identification @ STAR Gerd J. Kunde, Yale

TPC EMC Future RPC

Summary


420 CM

TPC Gas Volume & Electrostatic Field Cage

• Gas: P10 ( Ar-CH4 90%-10% ) @ 1 atm

• Voltage : - 31 kV at the central membrane 148 V/cm over 210 cm drift path

Self supporting Inner Field Cage: Al on Kapton using Nomex honeycomb; 0.5% rad length


Solenoidal Magnetic Field

• Magnetic Field

0.0 G

2.5 kG

5.0 kG

• Radial Uniformity

40 gauss

• Phi Uniformity

1 gauss


Outer and Inner Sectors of the Pad Plane

60 cm

190 cmOuter sector6.2 × 19.5 mm pads

3940 pads

Inner sector2.85 × 11.5 mm pads

1750 pads

• 24 sectors (12 on a side)

• Large pads good dE/dx resolution in the Outer sector

• Small pads for good two track resolution in the inner sector


Au on Au Event at CM Energy ~ 130 GeV*A

Two-track separation 2.5 cm

Momentum Resolution < 2%

Space point resolution ~ 500 m

Rapidity coverage –1.5 < < 1.5

A Central Event

Typically 1000 to 2000 tracks per event into the TPC


Drift Velocity Control Using Lasers and Tracks

Pressure (mbar)

5.44

5.45

Drif

t ve

loci

ty (

cm/

s)

1010 1020

• Lasers for coarse value

• Fine adjustment from tracking matching both side of the TPC


Electric Field Distortions

• No wires at the boundary between the inner and outer sectors

– E field leak• E field radial component• ExB effect in R and

Outer sector Inner sector

Gating grid = -127 VGround plane = 0 V

1.6 cm

Pad row #10 20 30

Data

Res

idu

al (

mm

)

0.2

0.1

-0.1

0.

Res

idu

al (

mm

)

Calculation

gap

Inner sector Outer sector

Radius (cm)

Wieman, JT (LBNL), Long, Trentalange (UCLA)


Many Effects – B, E, Clock, Twist, CM …

Hui Long, Steve Trentelange(UCLA), JT (LBNL)

All calculated distortions

20

60

100

140

-200 2000-100 100

180

Z (cm)

Rad

ius

(cm

)

Distortion scale 1.5 mm

Outer sectorInner sector

Tra

ck R

esid

ual

s (c

m) Before

> 200 m

Inner sector

Outer sector

Tra

ck R

esid

ual

s (c

m) After

< 50 m


Summary of Performance Achieved to Date• Good particle separation using dE/dx

– 7.5% dE/dx resolution– -proton separation : > 1 GeV/c

• Position resolution– 500 m– Function of dip angle and crossing angle

• 2-Track resolution– 2.5 cm

• Momentum resolution– 2%

• Unique features of the STAR TPC– 4 meter by 4 meter scale length– No field wires in the anode planes– Low gain– Very compact FEE electronics– Analog and Digital are not synchronous– Data delivered via optic fiber– Uniform B and E field– Distortions correctable to 50 m

• Lots of physics from the year 1 data – Collective flow– Identified particle spectra– Particle correlations– Event by event physics– Strangeness

• Future challenges– Achieve turn-key operation– Handle increased luminosity …


Offline Particle Identification by dE/dx

6.7%Design

7.5%With calibration

9 %No calibration

12

Kp

d

ed

E/d

x (k

eV

/cm

)

0

8

4

Anti - 3He

dE/dx PID range:

~ 0.7 GeV/c for K/

~ 1.0 GeV/c for K/p


Particle ID via Topology & Combinatorics

Secondary vertex: Ks + p +

+ + K e++e-

Ks + + - K + + K- p + - + + -

from K+ K- pairs

K+ K- pairs

m inv

m inv

same event dist.mixed event dist.

background subtracted

dn/dm

dn/dm

“kinks”

K +


STRANGENESS! (Preliminary)

K0

s

K+

bar


Two Photon Decays

0 Branching Ratio 98.80 % Z Z ee++ e e-- Z Z Conversion Probability ~ 1%• ee++ and and ee-- Tracking Efficiency 60 - 90% • Overall 0 Reconstruction probability ~ 10-4

0

e+

e- e+

e-

Z Z ee++ e e-- Z Z


Pizero Reconstruction

r (c

m)

100

z (cm)0-100-200 1000

TPC

TPC

4 Primary Photon Candidates

Primary Vertex

e-

e+

Z

Z Z ee++ e e-- Z

Detected energy

loss in the TPC

B.

Note: Most tracks are not shown


xy distance of closest approach (cm)

z distance of closest approach (cm)

• Opposite charged tracks• Small distance of closest

approach• Small opening Anglee-

e+

e+

e-

coun

ts

Topological Selection

B.

positron – electron


Primary Photon Selection

p = pe+ + pe-

Primary Vertex

Photon Momentum Vector

e+

e-

Photon Conversion Vector

Angular Difference

coun

ts

Angular Difference (Degrees)

4000

2000

6000

2100


Photon Purity: via positron dE/dx

photon pt (Gev/c)

Pur

ity69%

-4 0 -4

1

0e+ dE/dx deviant

p (GeV/c)10.1

dE/d

x

dE/d

x

5

2electron p(GeV/c)10

Conversion products

Minimum biasCentral

>95% below

1Gev/c

-bin the deviant in photon pt

-fit with a Gaussian+exp.

Applyphoton

cuts


Extracting YieldsOne photon

rotated by in ,2nd order

polynomial

Two photon invariant mass spectrum,

Gaussian + Nbg*(2nd poly)

After background subtraction,Gaussian

0


x-ray like images

• Photon conversion points– Conversion probability, (lZ2)

• A tool not ‘just’ physics– map the detector material– improve the material layout in

Geant

Inner Field Cage

SVTGlue joints

Mc data

Real data


STAR Barrel Electromagnetic Calorimeter (BEMC)

• Full barrel EMC– -1.0 < < 1.0– Full azimutal coverage– 120 modules

• ()module ~ (1.0, 0.1)• 40 towers/module

– 21 X0

– ()tower ~ (0.05, 0.05)– E/E ~ 14%/√E

• Shower max detector– Positioned at ~ 5 X0

– Larger spatial resolution – () ~ (0.007, 0.007)

• Pre-shower detector– 2 X0

– not avaliable this year


BEMC patch for next run

• Full West side– 60 modules

• 2400 towers• 18 K SMD channels

• Huge impact on physics

– High-pt 0

– electrons– Jets– J/


BEMC calibration – MIP peak

• Select MIP candidate– Low multiplicity event– Vertex cut to keep tower

projective characteristics– Track momentum > 1.2

GeV/c– The projection of the track in

the inner and outer EMC radius must be in the same tower

– All adjacent towers shall not have any projected tracks

• Peak + background fit• Mean ADC gain from MIP peak

position– 8 MeV/ADC count

220 k minibias AuAu events|zvertex| < 40 cmpMIP > 1.2 GeV/c


BEMC High tower trigger performance

• Threshold set at 2 GeV – Big enhancement at

high pt tracks (~30 at 6-7 GeV/c)

– Enhancement at way side tracks (back-to-back jets?)


0 reconstruction with BEMC

• 0 in AuAu events– 200 k minibias events– No SMD present

• Only towers• Larger background• Small shift on mass

value

STAR preliminary


e/h discrimination with BEMC

• Neural network software under development

• 5 parameters– Etower/ptrack

– EPSD

– ESMD

– Width of point ()– Separation between point

and projected track ()• Hadronic suppression

becomes worse without PSD – Simulations are under

way


Ranges of Particle Identification


Resistive Plate Chambers

• Narrow single gaps don’t work well in avalanche mode • Wider single gaps?

– enhanced streamer-free range of operating voltage • but time resolution suffers...

– primary ionziation is a stochastic process!• timing jitter from location of ionization in RPC

– avalanches from single primary clusters tend to merge• fluctuations in avalanche development dominate

• Many narrow gaps!– characteristic distance for primary ionization decreased

• decreased timing jitter from primary ionization step– N-independent avalanches, hence an averaging

• decreased timing jitter from avalanche fluctuations


Comparison


Alice Prototype


Rice Final Prototype


rpc


Proposal to install 60 m2 in STAR


Production of RPC at Rice


FEE Breakthrough


RPC Performance

Voltage (kv) Voltage (kv)

TimeEff.

Eff.

Time

cell 5 cell 11 (final)


RPC Summary

• TOF remains a viable technique for Particle Identification in modern experiments...

• MGRPC detectors are inexpensive and appear to outperform the conventional technology...

• Recent Major Successes a specific fishing line is a great choice for the 220 µm spacer...

• Detector module design (Rice v.11) is now final for STAR... <60ps stop-resolution is typical...

• Maxim 3760 preamp & other standard components is an adequate approach to the FEE...

• Collaboration of US and Chinese institutions developed...


Summary

• TPC– Identification via dE/dx

– Topological Methods/Combinatorical Methods

– Pizero

– Baryons up to 5 GeV/c

• EMC– Pizero

– Hadron Suppression

• RPC– Prototype with <60ps Stop Resolution

– Proposal for 60 m2


STAR Collaborators/Institutions

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan

England:

University of Birmingham

France:

Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany:

Max Planck Institute Munich, University of Frankfurt

India: Institute of Physics - Bhubaneswar, VECC

Calcutta, Panjab University - Chandrigrarh, University of Rajasthan - Jaipur, Jammu University, IIT -Bombay

Poland:

Warsaw University, Warsaw University of Technology

Russia:

MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP - Protvinoh

U.S. Universities:

Arkansas, UC Berkeley, UC Davis, UCLA,

Carnegie Mellon, Creighton, Indiana, Kent

State, MSU, CCNY, Ohio State, Penn State,

Purdue, Rice, Texas A&M, UT Austin,

Washington, Wayne State, Yale

U.S. Labs:

Argonne, Berkeley, and Brookhaven

National Laboratories

~400

Particle Identification @ STAR

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