8 Feb. 2005 1

Photon measurements in forward rapidity

Y.P. ViyogiVariable Energy Cyclotron Centre, Kolkata

ICPAQGP-2005

Kolkata, 8-12 Feb. 2005

8 Feb. 2005 2

Why measure photons in forward rapidity ?

Measuring photons in the forward rapidity is a challenge in itself.

Since photons come mostly from pi-0 decay, they complement the data on identified charged pion measurements.

In heavy ion experiments, it is important to compare similar results from different particles. It becomes necessary to measure photon production in as much detail as possible as they are the only major neutral particle amenable to detection.

8 Feb. 2005 3

Photon Multiplicity Detector (PMD)

PMD : a preshower detector measuring spatial distribution of photons in the forward rapidity region

Supplements the study of photons in the region where calorimeter cannot be used due to high particle density. Hoever, pT measurement is not poissible

PMD probes thermalisation (flow),

phase transition (multiplicity fluctuation),Chiral symmetry restoration (charged-neutral fluctuation)

pT Acceptance of PMD ~ 30 MeV/c (estimated)

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PMD@SPS

Plastic Scintillator pads with wavelength shifting fibres read out using image intensifier + CCD camera systems.

3 X0 thick Lead converter

Scintillator pads of size: 15, 20, 23, 25 mm2

Building blocks of PMD

•WA93 (S + Au, 200AGeV, 1991-92) : 8000 pads in 3m2 covering 2.8 < < 5.3

NIM A372 (1996) 143

•WA98 (Pb + Pb, 158AGeV, 1994-96): 53000 pads in 21m2 covering 2.9 < < 4.2

NIM A424 (1999) 395

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PMD in the WA93 Experiment

Bundle of 1900 fibres for one readout camera

Fiducial fibres2.8 < < 5.3

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WA98 Experiment at CERN SPS

Each box has 1900 pads, Is read out by one II + CCD camera

28 Box modules,53000 pads

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PMD @ colliders

At RHIC (STAR) and LHC (ALICE) experiments :

PMD in forward region : Detector design criteria

Confine charged particle hits to single cell to restrict occupancy and

improve photon-hadron discrimination;

Reduce cross-talk : stop -rays within a cell;

Use neutron-insensitive gas mixture (Ar +CO2)

Copper honeycomb

Closest to circular geometry

Excellent packing

Wall to stop -electrons

Cathode at high negative potential

Anode wire at ground, connected to readout

,.

Cathode extension

A small section

PCB

A unit cell

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PMD in the STAR Experiment

Cell cross section : 1.0 cm2

Cell depth : 0.8 cmTotal number of cells : 82,944 Area of the detector : 4.2 m2

Distance from vertex : 540 cm Coverage: -2.3 to -3.8 in with full

NIM A499 (2003) 751

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PMD in ALICE @ LHC

PMDZ~360cm

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PMD in ALICE

, coverage 2.3-3.5, 2Distance from IP 361.5 cm

Cell cross-section 0.22 cm2

Cell depth 0.5 cm

Total no. of cells 221184

Unit Module, 4608 cells

NIM A488 (2002) 131

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Results from PMD

1. Pseudorapidity distributions

2. Charged Neutral Fluctuation

3. Flow

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Photon -distributions at the SPS

S+Au, 200A.GeV

Phys. Rev. C58 (1998)1146

Pb+Pb, 158A.GeV

Phys. Lett. B458 (1999) 422

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Photon -distributions at the RHIC

Data : PMD in STARAu+Au, 63.A GeV

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Energy dependence of Limiting fragmentation scenario

- ybeam

dN

/d

part

See also talk by B. Mohanty in parallel session on 10/02 afternoon

STAR Collaboration, paper submitted to PRL, 4 Feb. 2005

Photon production follows the limiting fragmentation

scenario

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Centrality dependence of Limiting fragmentation scenario

Photon production follows centrality independent limiting fragmentation scenario

Charged particles follow centrality dependent limiting fragmentation scenario

- ybeam

dN

/d

part

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Limiting fragmentation for identified mesons

y - ybeam

dN

/d

part

Data NA49 Nch at various energiesBRAHMS Nch at 200.A GeV

STAR N0 (scaled N) at 63.A GeV

Limiting FragmentationCentrality dependent for

Inclusive charged particlesCentrality independent for

identified pions

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Chiral Symmetry Restoration : DCC

Look at N vs. Nch fluctuations

At T > Tc : Chiral symmetry restored

Vacuum expectation value of chiral field is zero.

At T < Tc : Chiral symmetry broken

Vacuum may be oriented in one of the pion directions (disoriented wrt normal vacuum directions)

Disoriented chiral condensates (DCC) formed in domains of (η,φ) : emission of low pt pions

Distribution of neutral pion fraction (ƒ) very different for DCC and generic events

π° 2 : shows up in photon detectors

π : shows up in charged particle detectors

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N vs. Nch Fluctuation• Top 5% central events ONLY• Bins in

• Discrete Wavelet Analysis

• Correlation Analysis:

NormalDCC

FFC

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Sensitivity to DCC Simulation with a simple DCC model : π°/ π± introduced at freezout

Mixed events for PMD/SPMD

Breaks different correlations (detector effects)

M1 : both individually mixed

M2 : Nγ and Nch from different events

M3γ : PMD no, SPMD mixed

M3ch : SPMD no, PMD mixed

nDCC : event sample with some fraction of DCC events

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Formation of DCC – upper limits

0-5% central

Global DCC

Upper limit for DCC-like localized fluctuations: 1% - 0.3% for central collisions for domains of size 45°- 60° within common -coverage.

Localized DCC domain

= fraction of pions as DCC pions

Phys. Lett. B420 (1998) 169Phys. Rev. C67 (2003) 044901

0-5% central

5-10% central

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Charged particle depleted events

Anti-CENTAURO event of JACEE36 photons, 1 charged particle

An event of WA98 PMD – SPMD

84 photons, 12 charged particle=2.9-3.75, =90°

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Inspection of event structure

• Scan the entire azimuthal range by

opening a window • Gradually slide the window by 2°

• Calculate ƒ = N*0.5/(N*0.5+Nch)

for each window.

• Find maximum value of ƒ in an event

represented by ƒmax .

Δη

Δφ patch

Details of SWM : See Poster by M.M. Aggarwal et al.

Sliding Window Method (SWM)

PMD-SPMDOverlap zone

Study photon-excess (exotic) regions, f > 0.55 higher purity of photon sample

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Neutral fraction distribution

Randomly selected patchesMean = 0.342, Sigma = 0.046

ƒmax in data extends to ~ 0.7, ~8 away from mean of ƒ

150K events, top 10% centrality

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Checking instrumental problems

Exotic patches uniformly distributed in azimuthDetector artifacts ruled out after various studies

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Exotic patches > 60°

After selecting a patch of = 60°, window size is increased in steps of 2° on both sides till ƒ remains > 0.55

Suggesting the presence of many patches of larger sizes

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Statistical Significance

Scatter plot of N and Nch differences for exotic and normal patches in exotic events

Mostly positive N and negative Nch differences

eN - N

n

nN

ch -

Nch

e

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Non-statistical fluctuation in f

N-Nch

N = ------------ (N+Nch )½

Genuine photon excess and depletion of charged particles beyond statistical fluctuations

Normal patches, peak ~ 0.35

Exotic patches, peak ~ 4.5

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Comparison of Data with Mixed and V+G Events

It is observed that events with large ƒ are more frequent in data as compared to those seen in mixed and V+G events.

Patches with ƒ > 0.55, which are 4.5 away from mean of ƒ distribution have been labeled as ‘exotic’ patches.

Percentage of events having patches with ƒmax > 0.55

Case Percentage Data 0.39±0.016(stat.)+0.17(syst.)-0.20(syst.) Mixed 0.081±0.007(stat.) V+G 0.013±0.008(stat.)

All SWM results of WA98 expt. are preliminary

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Charged-neutral correlation in STAR

PMD behind Forward TPC, which measures charged particle pT

Cut on pt (ch) greatly enhances the strength

χ

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Thermalisation : Flow

---dφ

~ 1 + 2 Σ vn cos nφ

v1 : shift of centroid, v2 : measure of ellipticity

Ψ1 , Ψ2 : specify orientation

n=1 : directedn=2 : elliptic

Pressure gradient in the overlap zone

collective flow in the reaction plane

Initial space anisotropy

Carried to final state momentum anisotropy

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Azimuthal Anisotropy : WA93 PMD

First observation ofCollective Flow at SPS Energy

Second order anisotropy coeff. (elliptic flow) of photons

v2

Contribution from π°decay

Phys. Lett. B403 (1997) 390

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Anisotropy in Neutral Pions

-------- = ---------- + c, Vin(π°) (χ –b)²

Scaling relation

Simulation of a large number of data set for various combinations of flow and

multiplicity

Parameter m can be determined from experimental data

Constants a,b,c depend on the order of anisotropy

Phys. Lett. B489 (2000) 9.

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Directed and Elliptic Flow of Charged particles (WA98 SPMD)

Syst. Err. Due toVertex shift

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Energy dependence of elliptic flow

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Photon Flow (WA98 PMD)

Simulation : use pi-0 flow = charged particle flow, include decay and kinematics. Shaded regions indicate simulation uncertainties.

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Comparison of PMD and LEDA v2

Paper submitted to EPJ , See also Poster by Raniwala et al.

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Possibilities at the LHC

PMD in ALICE extends from =2.3 to =3.5, depending on the pseudorapidity density, it should intercept 1000-3000 particles. One can study anisotropy with an accuracy of better than 3%.

Event plane determined from the PMD can be used to study correlations with other observables.

PMD has complete overlap with FMD, the charged particle multiplicity detector in the forward region. Should permit the study of charged neutral fluctuations.

Non-statistical fluctuation in multiplicity, to the level of 2% or more, should be observable in the PMD.

Source : PMD TDRCERN/LHCC 99-32, CERN/LHCC 2003-038

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Summary

It is important to measure photon production in as much detailand in as much extended part of phase space as possible. The results at RHIC show that these measurements arecomplementing the identified pion data.

Measuring photon multiplicity is important for the study ofcharged-neutral fluctuation. Preliminary results from WA98Expt. are quite interesting. This can be studied further both at RHIC (STAR) and at LHC (ALICE).

PMD will remain an important detector component to studyanisotropy and flow even at LHC.

8 Feb. 2005 39

The PMD Team

WA93VECC, Chandigarh, Jaipur, Jammu, GSI

WA98VECC, Bhubaneswar, Chandigarh, Jaipur, Jammu, GSI

STARVECC, Bhubaneswar, Chandigarh, Jaipur, Jammu, IITB

ALICEVECC, Bhubaneswar, Chandigarh, Jaipur, Jammu, IITB, BARC

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Purity vs.f

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(I) Time-specific detectors’ malfunctioning - Examining immediate preceding and succeeding events

Application to ….

Sept 21, 2004 Physics Forum,ALICE WEEK

Checks to rule out detector artifacts

ƒ distribution peaks around 0.35, similar to that of generic events

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Application to ….

Sept 21, 2004 Physics Forum,ALICE WEEK

Checks to rule out detector artifacts

(II) Distribution of exotic patches in azimuth

Uniformly distributed

8 Feb. 2005 43

Application to ….

Sept 21, 2004 Physics Forum,ALICE WEEK

Checks to rule out detector artifacts

(III) Nch distribution in non-overlapping region with PMD in exotic events

Exotic and Normal are quite similar

(2.35 < < 2.9)

I – III Suggest the normal behaviour of detectors