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Enhanced Production of Direct Photons in Au+Au Collisions at sqrt(sNN)=200 GeV and Implications for the Initial Temperat

This study investigates the production of direct photons in Au+Au collisions at RHIC energies and its implications for the initial temperature of the matter formed. Thermal photons and photons from hard scattering processes are considered as potential sources. The measurement of direct photons is challenging due to the dominance of photons from hadron decays. A method using low-mass lepton pairs as a proxy for direct photons is proposed to reduce the hadron decay background.

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Enhanced Production of Direct Photons in Au+Au Collisions at sqrt(sNN)=200 GeV and Implications for the Initial Temperat

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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 the PHENIX Collaboration APS “April Meeting” @ Washington DC February 15, 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. QCD Phase Transition • The colliding nuclei at RHIC energies would melt from protons and neutrons into a collection of quarks and gluons • Recreate the state of Universe a few microcse after the Big Bang Quark Gluon Plasma Hadron Tc ~ 170 MeV; e ~ 1 GeV/fm3 Measure the initial temperature of matter formed at RHIC Is Tinit higher than Tc ~ 170 MeV?

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

  5. Photon Probe of Nuclear Collisions g g p p f Time p K freeze-out time expansion Hadron Gas quark-gluon plasma hard parton scattering Space Au Au p p K Photons can probe the early stage of the reaction deep inside of the dense matter

  6. Many source of photons p p r g pQCD direct photons from initial hard scattering of quarks and gluons quark gluon g Thermal photonsfrom hot quark gluon plasma Thermal photonsfrom hadron gasafter hadronization background g Decay Photons from hadrons (p0, h, etc) p g

  7. Thermal photons (theory prediction) g q g q p p r g Hadron decay photons 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 difficult since the expected signal is only 1/10 of photons from hadron decays

  8. Direct Photons in Au+Au Blue line: Ncoll scaled p+p cross-section Direct photon is measured as “excess” above hadron decay photons Measurement below pT<3 GeV/c is 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

  9. Alternative method --- measure virtual photons e- e+ • 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 • Cost: the yield is reduced by a large factor (~ a/3p ~ 1/1000)

  10. 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 measure 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.

  11. 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

  12. Theory prediction of (Virtual) photon emission Theory calculation by Ralf Rapp Real photon yield Turbide, Rapp, Gale PRC69,014903(2004) at y=0, pt=1.025 GeV/c The calculation is shown as the virtual photon emission rate. The virtual photon emission rate is a smooth function of mass. When extrapolated to M=0, the real photon emission rate can be determined. q+gq+g* is not in the calculation; 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

  13. 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

  14. 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

  15. e+e- mass spectra in pT slices p+p Au+Au A.Adare et al. arXiv:0912.0244 • p+p in agreement with cocktail • Au+Au low mass enhancement concentrated at low pT

  16. Enhancement of almost real photon pp Au+Au (MB) • Low mass e+e- pairs (m<300 MeV) for 1<pT<5 GeV/c • p+p: • Good agreement with hadronic decay cocktail • Small excess above mp due to pQCD direct g at high pT • Au+Au: • Clear enhancement visible above mp =135 MeV for all pT Mp Mp 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV A. Adare et al., PRL accepted

  17. 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

  18. 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 A. Adare et al., PRL accepted

  19. Direct measurement of S(mee, pT) Au+Au 200 GeV Vaccuum HMBT @ pt=1.025 GeV/c Drop mass qq 0.1 0.2 0.3 0.4 0.5 0.6 0.7 GeV 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 A.Adare et al. arXiv:0912.0244

  20. 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 p+p Au+Au (MB) NLO pQCD μ = 0.5pT μ = 1.0pT μ = 2.0pT NLO pQCD calculation by Werner Vogelsang A. Adare et al., PRL accepted

  21. Direct photon spectra exp + TAA scaled pp A. Adare et al., PRL accepted • 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: TAuAu = 221  19stat  19syst MeV • Theoretical prediction of thermal photon by Turbide et al. agrees with the data within about a factor of two. Fit to pp NLO pQCD (W. Vogelsang)

  22. 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 • TAuAu is time-averaged “effective” temperature

  23. Theory comparison A.Adare et al. arXiv:0912.0244 • 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 agree with the data within a factor of ~2

  24. Initial temperature A.Adare et al. arXiv:0912.0244 TAuAu(fit) ~ 220 MeV TC from Lattice QCD ~ 170 MeV From data: Tini > TAuAu ~ 220 MeV From models: Tini = 300 to 600 MeV for t0 = 0.15 to 0.6 fm/c Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV

  25. On the Map “free” Gas We are here “free” Gas 500 Plasma 400 T (MeV) Quark Gluon Plasma “Perfect” Liquid 300 “Perfect” Liquid 200 Hadrons Tc ~ 170 MeV; e ~ 1 GeV/fm3 100 At these temperature, QGP is “perfect” liquid. At higher temperature, it can become “gas”

  26. Outlook signal electron e- partner positron needed for rejection Cherenkov blobs e+ qpair opening angle ~ 1 m 26 A new detector, HBD, was installed in PHENIX. HBD will greatly improve e+e- pair measurements, including the virtual photon analysis. We are now taking Au+Au data with HBD in RUN10 CsI photocathode covering triple GEMs Removes background e+e- pairs HBD: novel windowless Cerenkov detector with CF4 gas (radiator/working gas)

  27. 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 p+p is consistent with 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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