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

Photon measurements in forward rapidity Y.P. Viyogi Variable Energy Cyclotron Centre, Kolkata. ICPAQGP-2005 Kolkata, 8-12 Feb. 2005. Why measure photons in forward rapidity ?. Measuring photons in the forward rapidity is a challenge in itself.

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

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  1. Photon measurements in forward rapidityY.P. ViyogiVariable Energy Cyclotron Centre, Kolkata ICPAQGP-2005 Kolkata, 8-12 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.

  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 pT Acceptance of PMD ~ 30 MeV/c (estimated) PMD probes thermalisation (flow), phase transition (multiplicity fluctuation), Chiral symmetry restoration (charged-neutral fluctuation)

  4. 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 • 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 Building blocks of PMD

  5. Bundle of 1900 fibres for one readout camera Fiducial fibres PMD in the WA93 Experiment 2.8 <  < 5.3

  6. WA98 Experiment at CERN SPS 28 Box modules, 53000 pads Each box has 1900 pads, Is read out by one II + CCD camera

  7. 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) 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

  8. 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

  9. PMD in ALICE @ LHC PMD Z~360cm

  10. PMD in ALICE Unit Module, 4608 cells , coverage2.3-3.5, 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

  11. Results from PMD • Pseudorapidity distributions • Charged Neutral Fluctuation • Flow

  12. Pb+Pb, 158A.GeV S+Au, 200A.GeV Phys. Rev. C58 (1998)1146 Phys. Lett. B458 (1999) 422 Photon -distributions at the SPS

  13. Photon -distributions at the RHIC Data : PMD in STAR Au+Au, 63.A GeV

  14. h - ybeam 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

  15. 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

  16. 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

  17. Chiral Symmetry Restoration : DCC 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 Look at Ng vs. Nch fluctuations

  18. Normal DCC FFC • Top 5% central events ONLY • Bins in f : 1,2, 4, 8, 16 • Discrete Wavelet Analysis • Correlation Analysis: Ngvs. Nch Fluctuation

  19. 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

  20. 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.

  21. 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

  22. Δφ patch Δη Inspection of event structure Sliding Window Method (SWM) • 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 . PMD-SPMD Overlap zone Study photon-excess (exotic) regions, f > 0.55 higher purity of photon sample Details of SWM : See Poster by M.M. Aggarwal et al.

  23. Neutral fraction distribution 150K events, top 10% centrality Randomly selected patches Mean = 0.342, Sigma = 0.046 ƒmax in data extends to ~ 0.7, ~8 away from mean of ƒ

  24. Checking instrumental problems Exotic patches uniformly distributed in azimuth Detector artifacts ruled out after various studies

  25. 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

  26. e n Ng - Ng e n Nch - Nch Statistical Significance Scatter plot of N and Nch differences for exotic and normal patches in exotic events • Mostly positive N and negative Nch differences

  27. 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

  28. 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.) 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 All SWM results of WA98 expt. are preliminary

  29. χ Charged-neutral correlation in STAR PMD behind Forward TPC, which measures charged particle pT Cut on pt (ch) greatly enhances the strength

  30. Thermalisation : Flow Initial space anisotropy  Carried to final state momentum anisotropy Pressure gradient in the overlap zone collective flow in the reaction plane n=1 : directed n=2 : elliptic ~ 1 + 2 Σ vn cos nφ dN --- dφ v1 : shift of centroid, v2 : measure of ellipticity Ψ1 , Ψ2 : specify orientation

  31. v2 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 π° decay

  32. 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( γ ) a -------- = ---------- + c, Vin(π°) (χ –b)²

  33. Directed and Elliptic Flow of Charged particles (WA98 SPMD) Syst. Err. Due to Vertex shift

  34. Energy dependence of elliptic flow

  35. Photon Flow (WA98 PMD) Simulation : use pi-0 flow = charged particle flow, include decay and kinematics. Shaded regions indicate simulation uncertainties.

  36. Comparison of PMD and LEDA v2 Paper submitted to EPJ , See also Poster by Raniwala et al.

  37. 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

  38. 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.

  39. 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

  40. Purity vs.f

  41. Application to …. Checks to rule out detector artifacts (I) Time-specific detectors’ malfunctioning -Examining immediate preceding and succeeding events • ƒ distribution peaks around 0.35, similar to that of generic events Sept 21, 2004 Physics Forum,ALICE WEEK

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

  43. Application to …. Checks to rule out detector artifacts (III) Nchdistribution in non-overlapping region with PMD in exotic events (2.35 <  < 2.9) Exotic and Normal are quite similar • I – III Suggest the normal behaviour of detectors Sept 21, 2004 Physics Forum,ALICE WEEK

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