Photon measurements in forward rapidity Y.P. Viyogi Variable Energy Cyclotron Centre, Kolkata

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Photon measurements in forward rapidityY.P.
ViyogiVariable Energy Cyclotron Centre, Kolkata
ICPAQGP-2005 Kolkata, 8-12 Feb. 2005
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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.
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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
pT Acceptance of PMD 30 MeV/c (estimated)
PMD probes thermalisation (flow), phase
transition (multiplicity fluctuation), Chiral
symmetry restoration (charged-neutral fluctuation)
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PMD_at_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

• WA93 (S Au, 200AGeV, 1991-92)
• 8000 pads in 3m2 covering 2.8 lt ? lt 5.3
• NIM A372 (1996) 143
• WA98 (Pb Pb, 158AGeV, 1994-96)
• 53000 pads in 21m2 covering 2.9 lt ? lt 4.2
• NIM A424 (1999) 395

Building blocks of PMD
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PMD in the WA93 Experiment
2.8 lt ? lt 5.3
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WA98 Experiment at CERN SPS
28 Box modules, 53000 pads
Each box has 1900 pads, Is read out by one II
CCD camera
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PMD _at_ 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)
A unit cell
Copper honeycomb Closest to circular
geometry Excellent packing Wall to stop
?-electrons Cathode at high negative
potential Anode wire at ground, connected to
readout
A small section
,.
PCB
Cathode extension
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PMD in the STAR Experiment

• Cell cross section 1.0 cm2
• Cell depth 0.8 cm
• Total 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 _at_ LHC
PMD Z360cm
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PMD in ALICE
Unit Module, 4608 cells
?,? coverage 2.3-3.5, 2? Distance from IP 361.5
cm Cell cross-section 0.22 cm2 Cell depth 0.5
cm Total no. of cells 221184
NIM A488 (2002) 131
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Results from PMD

• Pseudorapidity distributions
• Charged Neutral Fluctuation
• Flow

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Photon ?-distributions at the SPS
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Photon ?-distributions at the RHIC
Data PMD in STAR AuAu, 63.A GeV
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Energy dependence of Limiting fragmentation
scenario

• Photon production follows
• the limiting fragmentation
• scenario

dN/dh / 0.5 Npart
See also talk by B. Mohanty in parallel session
on 10/02 afternoon STAR Collaboration, paper
submitted to PRL, 4 Feb. 2005
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Centrality dependence of Limiting fragmentation
scenario
dN/dh / 0.5 Npart
h - ybeam

• Photon production follows centrality independent
  limiting fragmentation scenario
• Charged particles follow centrality dependent
  limiting fragmentation scenario

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Limiting fragmentation for identified mesons
Data NA49 Nch at various energies BRAHMS Nch at
200.A GeV STAR N?0 (scaled N?) at 63.A GeV
dN/dh / 0.5 Npart
Limiting Fragmentation Centrality dependent
for Inclusive charged particles Centrality
independent for identified pions
y - ybeam
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Chiral Symmetry Restoration DCC
At T gt Tc Chiral symmetry restored ? Vacuum
expectation value of chiral field is zero. At T lt
Tc Chiral symmetry broken ? Vacuum may be
oriented in one of the pion directions
(disoriented wrt normal vacuum directions) Disorie
nted chiral condensates (DCC) formed in domains
of (?,f) emission of low pt pions Distribution
of neutral pion fraction () very different for
DCC and generic events p ? 2? shows up in
photon detectors p? shows up in charged
particle detectors
Look at Ng vs. Nch fluctuations
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Ng vs. Nch Fluctuation

• Top 5 central events ONLY
• Bins in f 1,2, 4, 8, 16
• Discrete Wavelet Analysis
• Correlation Analysis

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Sensitivity to DCC
Simulation with a simple DCC model p/ p
introduced at freezout
Mixed events for PMD/SPMD Breaks different
correlations (detector effects) M1 both
individually mixed M2 N? and Nch from different
events M3? 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
Global DCC
Localized DCC domain
5-10 central
0-5 central
0-5 central
? fraction of pions as DCC pions
Phys. Rev. C67 (2003) 044901
Phys. Lett. B420 (1998) 169
Upper limit for DCC-like localized
fluctuations 1 - 0.3 for central collisions
for domains of size 45- 60 within common
?-coverage.
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Charged particle depleted events
An event of WA98 PMD SPMD 84 photons, 12
charged particle ??2.9-3.75, ??90
Anti-CENTAURO event of JACEE 36 photons, 1
charged particle
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Inspection of event structure
Sliding Window Method (SWM)

• Scan the entire azimuthal range by
• opening a window ??
• Gradually slide the window by 2
• Calculate N?0.5/(N?0.5Nch)
• for each window.
• Find maximum value of in an event
• represented by max .

PMD-SPMD Overlap zone
Study photon-excess (exotic) regions, f gt 0.55
higher purity of photon sample
Details of SWM See Poster by M.M. Aggarwal et
al.
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Neutral fraction distribution
150K events, top 10 centrality
Randomly selected patches Mean 0.342, Sigma
0.046
max in data extends to 0.7, 8? away from
mean of
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Checking instrumental problems
Exotic patches uniformly distributed in
azimuth Detector artifacts ruled out after
various studies
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Exotic patches ?? gt 60

• After selecting a patch of ?? 60, window
  size is
• increased in steps of 2 on both sides till
  remains gt 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

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Non-statistical fluctuation in f
N?-Nch ?N ------------
(N?Nch )½

• Normal patches, peak 0.35
• Exotic patches, peak 4.5

• Genuine photon excess and depletion of charged
  particles
• beyond statistical fluctuations

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Comparison of Data with Mixed and VG Events
It is observed that events with large are more
frequent in data as compared to those
seen in mixed and VG events. Patches with gt
0.55, which are 4.5? away from mean of
distribution have been labeled as exotic
patches.
Percentage of events having patches with max gt
0.55
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
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Thermalisation Flow
Initial space anisotropy ?
Carried to final state momentum anisotropy
Pressure gradient in the overlap zone
collective flow in the reaction plane
n1 directed n2 elliptic
1 2 S vn cos nf
dN --- df
v1 shift of centroid, v2 measure of
ellipticity ?1 , ?2 specify orientation
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Azimuthal Anisotropy WA93 PMD
Second order anisotropy coeff. (elliptic flow) of
photons
First observation of Collective Flow at SPS
Energy
Phys. Lett. B403 (1997) 390
Contribution from p decay
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Anisotropy in Neutral Pions
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
Scaling relation
Phys. Lett. B489 (2000) 9.
V( ? ) a -------- ---------- c,
Vin(p) (? b)²
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Directed and Elliptic Flow of Charged particles
(WA98 SPMD)
Syst. Err. Due to Vertex 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 detail and in as much extended part of
phase space as possible. The results at RHIC
show that these measurements are complementing
the identified pion data.
Measuring photon multiplicity is important for
the study of charged-neutral fluctuation.
Preliminary results from WA98 Expt. are quite
interesting. This can be studied further both
at RHIC (STAR) and at LHC (ALICE).
PMD will remain an important detector component
to study anisotropy and flow even at LHC.
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The PMD Team
WA93 VECC, Chandigarh, Jaipur, Jammu,
GSI WA98 VECC, Bhubaneswar, Chandigarh, Jaipur,
Jammu, GSI STAR VECC, Bhubaneswar, Chandigarh,
Jaipur, Jammu, IITB ALICE VECC, Bhubaneswar,
Chandigarh, Jaipur, Jammu, IITB, BARC
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Purity vs.f
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Application to .
Checks to rule out detector artifacts
(I) Time-specific detectors malfunctioning
- Examining immediate preceding and
succeeding events

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

Sept 21, 2004
Physics Forum,ALICE WEEK
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Application to .
Checks to rule out detector artifacts
(II) Distribution of exotic patches in azimuth

• Uniformly distributed

Sept 21, 2004
Physics Forum,ALICE WEEK
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Application to .
Checks to rule out detector artifacts
(III) Nch distribution in non-overlapping
region with PMD in exotic events
(2.35 lt ? lt 2.9)
Exotic and Normal are quite similar

• I III Suggest the normal behaviour of
  detectors

Sept 21, 2004
Physics Forum,ALICE WEEK