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Direct photons at GSI

Direct photons at GSI. Sergey Kiselev, ITEP Moscow, for the ECAL group Introduction Prompt g g from transport codes Experimental method s Conclusions. Introduction - definitions.

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Direct photons at GSI

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  1. Direct photons at GSI • Sergey Kiselev, ITEP Moscow, for the ECAL group • Introduction • Prompt g • g from transport codes • Experimental methods • Conclusions CBM Collaboration meeting, Strasbourg S.Kiselev

  2. Introduction - definitions • In A+B collision direct photons - photons that originate during time of the collision, ctAB~ 50-100 fm; po, h,h’ rg/ωg/2g, have ct »ctAB - main origin of non-direct photons. • Quark gluon level: qq  gg, qg  qg, qq(g)  qq(g)g Initial hard NN collisions, pQCD  prompt g. Thermalised QGP stage  thermal g from QGP. • Hadron level: • decays: w  pg, a1  pg, D  Ng, K* Kg, f  hg, r  ppg, . . . • meson scatterings: pp  rg, pr  pg, pK  K*g, Kr  Kg, KK*  pg, pK*  Kg, . . . Thermalised hadron stage  thermal g from hadron gas. CBM Collaboration meeting, Strasbourg S.Kiselev

  3. Introduction – aims • To estimate for the CBM energy: • Prompt  contribution •  contribution from hadron sources (decays and rescatterings) • Possibility to explore the state-of-the-art experimental methods CBM Collaboration meeting, Strasbourg S.Kiselev

  4. Prompt g : yields at CBM • Data fit (xT>0.1): Ed3σpp/d3p = 575(√s)3.3 /(pt)9.14pb/GeV2 EPJ C22 (2001) 129 • If extrapolation to the CBM energy is correct one can estimate yields • ~10-4 prompt g with pt > 2 GeV/c per Au+Au central event at 25 AGeV • At beam intensity 109/s + 1% interaction + 10% centrality  prompt g rate100/s CBM Collaboration meeting, Strasbourg S.Kiselev

  5. Prompt g : PYTHIA PYTHIA subprocesses: q g  q γ, q qbar  g γ, q qbar  γγ Lowest c.m. energy: PARP(2)=7 instead of default 10. σ ~ 2 10-4 mb = ~85% (q g  q γ) + ~15% (q qbar  g γ) CBM Collaboration meeting, Strasbourg S.Kiselev

  6. g from transport codes • Photons from existing transport codes at the CBM energy (central Au+Au at 25 AGeV): No a transport code with direct photons! Need the meson scattering source of γ At pt > 1.5 GeV/c main contributions:ω πoγ, πρ  πγ CBM Collaboration meeting, Strasbourg S.Kiselev

  7. g from RQMD • RQMD (Relativistic Quantum Molecular Dynamics)Phys. Rev. C52 (1995) 3291 • 103 Au+Au central, b<3 fm, events at T/A=25 GeV have been generated • ≈5.4 g per event • 2/3 from h’ • 1/3 from w CBM Collaboration meeting, Strasbourg S.Kiselev

  8. g from HSD – meson scatterings Agreement with the HSD teamto implement πρ πγ, ππ ργ Kapusta et.al. (Phys.Rev.D44 (1991) 2774) : ITEP team: FORTRAN subroutines with cross sections were prepared for the HSD code HSD team: implementation of the cross sections into the HSD code CBM Collaboration meeting, Strasbourg S.Kiselev

  9. Experimental methods for direct photons • Last years: large progress in experimental techniques to extract direct photons • Direct photons have been revealed by methods: • Subtraction method, WA98/2000, PHENIX/2005 • Momentum correlation method, WA98/2004 • Internal conversion method, PHENIX/2005 • ? Real photon conversion method, PHENIX/2006 CBM Collaboration meeting, Strasbourg S.Kiselev

  10. Internal conversion - details Main assumption: the Kroll-Wada (KW) formula, Phys.Rev.98(1955)1355 A given reaction produces, instead of real γ, γ*  e+e-. For decays: Nγ – number of real γ πo γγπo γγ* γe+e- I=KW = 0.0059 η γγη γγ*  γe+e- I=KW = 0.0081 γdirectγ*direct e+e- I=KW = 0.0162 phase space factor =1, if peeT » mee But for a fixed target experimentthere is an additional source of e+e-in the A region from real γ conversion in a target The method must be modified for the fixed target experiment  CBM Collaboration meeting, Strasbourg S.Kiselev

  11. double ratio (arb. units) Real photon conversion • Hot Quark 2006: real photon conversion in the beam pipe, PHENIX, nucl-ex/0608009 • Clear γ e+e-identification • Very good, δp/p ~ 1%, of tracking system • While a result can not be presented yet (systematic ~25%), the method seems promising. CBM Collaboration meeting, Strasbourg S.Kiselev

  12. Conclusions • Prompt γ: reasonable agreement at pT~2 GeV/c between PYTHIA predictions and data extrapolation • γ from meson scatterings: cross sections for πρ πγππ ργ have been prepared, waiting for its implementation into the HSD code  direct photon dynamics at the CBM energy • Experimental methods: • Internal conversion method: modification for the fix target experiment • Real photon conversion method: no final result, QM’06 ? CBM Collaboration meeting, Strasbourg S.Kiselev

  13. Backup CBM Collaboration meeting, Strasbourg S.Kiselev

  14. Introduction – motivation • Data for excitation functions: • K+/+ ratios show a peak at ~30 A GeV (‚horn‘). • The kink in the K+slope. • Not reproduced by transport codes. The aim: to scan the energy region by other observable, direct photons. It is very difficult to extract direct photons. But high intensity beams and new experimental methods (SPS, RHIC) let us hope. CBM Collaboration meeting, Strasbourg S.Kiselev

  15. Quark gluon level CBM Collaboration meeting, Strasbourg S.Kiselev

  16. Prompt g : pp data • A Compilation of Data, J. Phys. G,23 (1997) A1 CBM can cover the range √s < 14 GeV CBM Collaboration meeting, Strasbourg S.Kiselev

  17. Prompt g : data fit • Data fit (xT>0.1): Ed3σpp/d3p = 575(√s)3.3 /(pt)9.14pb/GeV2 EPJ C22 (2001) 129 • For A+B: Ed3N/d3p(b) = Ed3σpp/d3p AB TAB(b) = Ed3σpp/d3p Ncoll/ σppin CBM Collaboration meeting, Strasbourg S.Kiselev

  18. g from hadron gas. An example • nucl-th/9712048, transport code based on the Walecka-typemodel At pt > 1.5 GeV/c main contributions: ω πoγ, πρ  πγ CBM Collaboration meeting, Strasbourg S.Kiselev

  19. g from UrQMD • UrQMD (Ultra Relativistic Quantum Molecular Dynamics) Prog. Part. Nucl. Phys. 41 (1998) 225–370. • Phys. Rev. C57 (1998) 3271 “Direct photons in Pb+Pb at CERN-SPS …” • The processes pp  rg, pr  pg were consideredexplicitly using cross sections given in Phys. Rev. D44, (1991) 2774. • the pr  pg and w  pg processes are dominant in the range1 GeV≤ kT ≤ 3 GeV. CBM Collaboration meeting, Strasbourg S.Kiselev

  20. g from UrQMD – at CBM • 103 UrQMD Au+Au central events at 25 AGeV • ≈ 14 photons per event. BUT all g are from decays, mainly a1  pg. Where are the processes pp  rg, pr  pg ? • 22 Feb 2006, M. Bleicher “. . . photons should not be calculated within theurqmd, but explicitely outside with a different code.everybody should ignore all processes with photons involved. we will move them out of the model in the next version.” CBM Collaboration meeting, Strasbourg S.Kiselev

  21. g from HSD • HSD (Hadron String Dinamics), Phys. Rep. 308 (1999) 65, 6.6. Direct photons p. 174 • BUT in the file generated for CBM, 103 central Au+Au events at 25 AGeV, there are NO photons at all !? CBM Collaboration meeting, Strasbourg S.Kiselev

  22. g from HIJING • HIJING (Heavy Ion Jet Interaction Generator) Phys. Rev. D 44, (1991) 3501. • Soft processes: FRITIOF • Hard processes: PYTHIA NO rescatterings • 25 AGeV: Error: too low CM energy, 6.982 GeV for event generation. Execution stopped! • BLOCK DATA PYDATA: PARP(2)=6.8 instead 10. • 103 Au+Au central, b<3 fm, events at T/A=25 GeV have been generated • 10.5 g per event • 60% from h’ • 36% from w • 4% from D • Though direct g production(IHPR2(3)=2) is included using PYTHIA, BUT at CBM energy the code does not generate direct g. CBM Collaboration meeting, Strasbourg S.Kiselev

  23. Subtraction method • Direct photon measurement by the subtraction method: WA98 PRL 85 (2000) 3595, PHENIX PRL 94 (2005) 232301 • WA98 subtracted from measured photons those from known hadronic source: from decays of reconstructedpo,h and other hadrons (with some assumption of its yield and spectrum) • At pt > 0,5 GeV/c • from po - 80% • from h - 13% • from w - 2% • from h’ - 1.3% CBM Collaboration meeting, Strasbourg S.Kiselev

  24. Subtraction method – cont. CBM Collaboration meeting, Strasbourg S.Kiselev

  25. Momentum correlationsmethod • ggmomentum correlations, WA98 PRL 93 (2004) 022301, STAR nucl-ex/0511055 • gg correlations direct photons • direct photons correlation provide the system sizes at all stages of heavy-ion collisions • Needs larger statistics • Rinv (gg) ≈ 6 fm ≈ Rinv (pp) • emitted in the late, hadron gas, stage of the collision • Thermalcalculations substantialy underestimate the data. CBM Collaboration meeting, Strasbourg S.Kiselev

  26. Low mass e+e- pairs • A novel technique (QM’05): internal conversion of direct photons into e+e-, PHENIX, nucl-ex/0511041 • any source of real g emits also virtual g with very low-mass • gdirect =( g*direct /g*incl.) gincl. Electrons in the central arms were identified by matching charged particle tracks to clusters in the ECAL and to rings in the RICH CBM Collaboration meeting, Strasbourg S.Kiselev

  27. Low mass e+e- pairs – cont. • Measurement for 1 < pT < 5 GeV/c consistent with calculations when thermal photon emission is taken into account (at high pT > 5 it is consistent with a NLO pQCD calculation). CBM Collaboration meeting, Strasbourg S.Kiselev

  28. CBM Collaboration meeting, Strasbourg S.Kiselev

  29. CBM Collaboration meeting, Strasbourg S.Kiselev

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