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Y. Akiba (RIKEN/RBRC) for PHENIX Collaboration January 19, 2010

Enhanced production of direct photons in Au+Au collisions at sqrt(s NN )=200 GeV and implications for the initial temperature. Y. Akiba (RIKEN/RBRC) for PHENIX Collaboration January 19, 2010. The PHENIX Collaboration. Abilene Christian University, Abilene, TX 79699, U.S.

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Y. Akiba (RIKEN/RBRC) for PHENIX Collaboration January 19, 2010

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  1. Enhanced production of direct photons in Au+Au collisions at sqrt(sNN)=200 GeV and implications for the initial temperature Y. Akiba (RIKEN/RBRC) for PHENIX Collaboration January 19, 2010

  2. The PHENIX Collaboration Abilene Christian University, Abilene, TX 79699, U.S. Collider-Accelerator Department, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S. Physics Department, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S. University of California - Riverside, Riverside, CA 92521, U.S. University of Colorado, Boulder, CO 80309, U.S. Columbia University, New York, NY 10027 and Nevis Laboratories, Irvington, NY 10533, U.S. Florida Institute of Technology, Melbourne, FL 32901, U.S. Florida State University, Tallahassee, FL 32306, U.S. Georgia State University, Atlanta, GA 30303, U.S. University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S. Iowa State University, Ames, IA 50011, U.S. Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S. Los Alamos National Laboratory, Los Alamos, NM 87545, U.S. University of Maryland, College Park, MD 20742, U.S. Department of Physics, University of Massachusetts, Amherst, MA 01003-9337, U.S. Muhlenberg College, Allentown, PA 18104-5586, U.S. University of New Mexico, Albuquerque, NM 87131, U.S. New Mexico State University, Las Cruces, NM 88003, U.S. Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S. RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S. Chemistry Department, Stony Brook University, Stony Brook, SUNY, NY 11794-3400, U.S. Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, NY 11794, U.S. University of Tennessee, Knoxville, TN 37996, U.S. Vanderbilt University, Nashville, TN 37235, U.S.

  3. Electromagentic probes (photon and lepton pairs) e+ e- g* g • Photons and lepton pairs are cleanest probes of the dense matter formed at RHIC • These probes have little interaction with the matter so they carry information deep inside of the matter • Temperature? • Hadrons inside the matter? • Matter properties?

  4. Thermal photon from hot matter Hot matter emits thermal radiation Temperature can be measured from the emission spectrum

  5. Thermal photons (theory prediction) g q g q p p r g S.Turbide et al PRC 69 014903 • High pT (pT>3 GeV/c) pQCD photon • Low pT (pT<1 GeV/c) photons from hadronic Gas • Themal photons from QGP is the dominant source of direct photons for 1<pT<3 GeV/c • Recently, other sources, such as jet-medium interaction are discussed • Measurement is difficut since the expected signal is only 1/10 of photons from hadron decays

  6. Direct Photons in Au+Au Blue line: Ncoll scaled p+p cross-section Direct photon is measured as “excess” above hadron decay photons Measurement at low pT difficult since the yield of thermal photons is only 1/10 of that of hadron decay photons PRL 94, 232301 (2005) Au+Au data consistent with pQCD calculation scaled by Ncoll

  7. Alternative method --- meaure virtual photon • Source of real photon should also be able to emit virtual photon • At m0, the yield of virtual photons is the same as real photon • Real photon yield can be measured from virtual photon yield, which is observed as low mass e+e- pairs • Advantage: hadron decay background can be substantially reduced. For m>mp, p0 decay photons (~80% of background) are removed • S/B is improved by a factor of five • Other advantages: photon ID, energy resolution, etc

  8. Not a new idea J.H.Cobb, et al, PL 78B, 519 (1978) C. Albajar, et al, PLB209, 397 (1988) g/p0 = 10% Dalitz g/p0 = 0.53 ±0.92% (2< pT < 3 GeV/c) The idea of measuring direct photon via low mass lepton pair is not new one. It is as old as the concept of direct photon. This method is first tried at CERN ISR in search for direct photon in p+p at s1/2=55GeV. They look for e+e- pairs for 200<m<500 MeV, and set one of the most stringent limit on direct photon production at low pT Later, UA1 measured low mass muon pairs and deduced the direct photon cross section.

  9. Relation between dilepton and virtual photon Emission rate of (virtual) photon e.g. Rapp, Wambach Adv.Nucl.Phys 25 (2000) Boltzmann factor temperature EM correlator Matter property Emission rate of dilepton Prob. g*l+l- Relation between them This relation holds for the yield after space-time integral Dilepton virtual photon Virtual photon emission rate can be determined from dilepton emission rate M ×dNee/dM gives virtual photon yield For M0, ng*  ng(real); real photon emission rate can also be determined

  10. Theory prediction of dilepton emission Theory calculation by Ralf Rapp at y=0, pt=1.025 GeV/c Usually the dilepton emission is measured and compared as dN/dptdM The mass spectrum at low pT is distorted by the virtual photonee decay factor 1/M, which causes a steep rise near M=0 qq annihilaiton contribution is negligible in the low mass region due to the M2 factor of the EM correlator In the caluculation, partonic photon emission process q+gq+g*qe+e- is not included Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+qg*ee (HTL improved) (q+gq+g*qee not shown) 1/M g*ee qqg*e+e- ≈(M2e-E/T)×1/M

  11. Virtual photon emission rate Real photon yield Turbide, Rapp, Gale PRC69,014903(2004) at y=0, pt=1.025 GeV/c The same calculation, but shown as the virtual photon emission rate. The steep raise at M=0 is gone, and the virtual photon emission rate is more directly related to the underlying EM correlator. When extrapolated to M=0, the real photon emission rate is determined. q+gq+g* is not shown; it should be similar size as HMBTat this pT Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+q annihilaiton (HTL improved) q+g  q+g* ? qqg* ≈M2e-E/T

  12. Electron pair measurement in PHENIX g p DC e+ e- PC1 magnetic field & tracking detectors PC3 designed to measure rare probes:+ high rate capability & granularity + good mass resolution and particle ID - limited acceptance • 2 central arms: electrons, photons, hadrons • charmonium J/, ’ -> e+e- • vector meson r, w,  -> e+e- • high pTpo, p+, p- • direct photons • open charm • hadron physics Au-Au & p-p spin

  13. LMR-I = quasi-real virtual photon • LMR I(pT >> mee)quasi-real virtual photon region. Low mass pairs produced by higher order QED correction to the real photon emission

  14. e+e- mass spectra in pT slices p+p Au+Au arXiv:0912.0244 • p+p in agreement with cocktail • Au+Au low mass enhancement concentrated at low pT

  15. Enhancement of almost real photon pp Au+Au (MB) arXiv:0804.4168 • Low mass e+e- pairs (m<300 MeV) for 1<pT<5 GeV/c • p+p: • Good agreement of p+p data and hadronic decay cocktail • Small excess above mp at large mee and high pT • Au+Au: • Clear enhancement visible above mp =135 MeV for all pT • Excess  Emission of almost real photon Mp Mp 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV

  16. Virtual Photon Measurement Any source of real g can emit g* with very low mass. Relation between the g* yield and real photon yield is known. Process dependent factor • Case of hadrons (p0, h) (Kroll-Wada) • S = 0 at Mee > Mhadron • Case of direct g* • IfpT2>>Mee2 S = 1 • For m>mp, p0 background (~80% of background) is removed • S/B is improved by a factor of five Direct g p0 h

  17. Determination of g*fraction, r Direct g*/inclusive g* is determined by fitting the following function fdirect : direct photon shape with S = 1. r = direct g*/inclusive g* • Fit in 120-300MeV/c2(insensitive to p0 yield) • The mass spectrum follows the expectation for m > 300 MeV •  S(m) ~ 1 arXiv:0804.4168 arXiv:0912.0244

  18. Direct measurement of S(mee, pT) Au+Au 200 GeV Vaccuum HMBT @ pt=1.025 GeV/c Drop mass qq No indication of mass dependence of R(m,pT) in this high pT region  S(m,pT) is near constant Extrapolation to M=0 should give the real photon emission rate R = (Data-cocktail)×Mee arXiv:0912.0244

  19. Fraction of direct photons Fraction of direct photons Compared to direct photons from pQCD p+p Consistent with NLO pQCD Au+Au Clear excess above pQCD arXiv:0804.4168 arXiv:0912.0244 p+p Au+Au (MB) μ = 0.5pT μ = 1.0pT μ = 2.0pT NLO pQCD calculation by Werner Vogelsang

  20. Direct photon spectra exp + TAA scaled pp arXiv:0804.4168 arXiv:0912.0244 • Direct photon measurements • real (pT>4GeV) • virtual (1<pT<5GeV) • pQCD consistent with p+p down to pT=1GeV/c • Au+Au data are above Ncoll scaled p+p for pT < 2.5 GeV/c • Au+Au = scaled p+p + exp: Tave = 221  19stat  19syst MeV Fit to pp NLO pQCD (W. Vogelsang)

  21. Summary of the fit • Significant yield of the exponential component (excess over the scaled p+p) • The inverse slope TAuAu = 221±19±19 MeV (>Tc ~ 170 MeV) • p+p fit funciton: App(1+pt2/b)-n • If power-law fit is used for the p+p spectrum, TAuAu = 240±21 MeV

  22. Theory comparison • Hydrodynamical models are compared with the data D.d’Enterria &D.Peressounko T=590MeV, t0=0.15fm/c S. Rasanen et al. T=580MeV, t0=0.17fm/c D. K. Srivastava T=450-600MeV, t0=0.2fm/c S. Turbide et al. T=370MeV, t0=0.33fm/c J. Alam et al. T=300MeV, t0=0.5fm/c F.M. Liu et al. T=370MeV, t0=0.6 fm/c • Hydrodynamical models are in qualitative agreement with the data

  23. Initial temperature Tave(fit) = 221 MeV TC from Lattice QCD ~ 170 MeV From data: Tini > Tave = 220 MeV From models: Tini = 300 to 600 MeV t0 = 0.15 to 0.6 fm/c Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV

  24. Summary and conclusion • We have measured e+e- pairs for m<300MeV and 1<pT<5 GeV/c • Excess above hadronic background is observed • Excess is much greater in Au+Au than in p+p • Treating the excess as internal conversion of direct photons, the yield of direct photon is dedued. • Direct photon yield in pp is consistent with a NLO pQCD • Direct photon yield in Au+Au is much larger. • Spectrum shape above TAA scaled pp is exponential, with inverse slope T=221 ±19(stat)±19(sys) MeV • Hydrodynamical models with Tinit=300-600MeV at t0=0.6-0.15 fm/c are in qualitative agreement with the data. • Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV

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