1. 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

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) 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?   e+e+ e-e-

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

5. Thermal photons (theory prediction) High p T (p T >3 GeV/c) pQCD photon Low p T (p T <1 GeV/c) photons from hadronic Gas Themal photons from QGP is the dominant source of direct photons for 1<p T <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 S.Turbide et al PRC 69 014903 q qg     

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

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>m ,  0 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 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 s 1/2 =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 p T Later, UA1 measured low mass muon pairs and deduced the direct photon cross section.  /  0 = 10% J.H.Cobb, et al, PL 78B, 519 (1978)  /  0 = 0.53 ±0.92% (2< p T < 3 GeV/c) Dalitz C. Albajar, et al, PLB209, 397 (1988)

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

10. Theory prediction of dilepton emission Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+q     ee (HTL improved) (q+g  q+    qee not shown) Theory calculation by Ralf Rapp at y=0, pt=1.025 GeV/c Usually the dilepton emission is measured and compared as dN/dp t dM The mass spectrum at low p T 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 M 2 factor of the EM correlator In the caluculation, partonic photon emission process q+g  q+    qe + e - is not included 1/M  *  ee qq   *  e + e - ≈(M 2 e -E/T ) × 1/M

11. Virtual photon emission rate at y=0, pt=1.025 GeV/c Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+q annihilaiton (HTL improved) 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+  * is not shown; it should be similar size as HMBT at this p T Real photon yield Turbide, Rapp, Gale PRC69,014903(2004) q+g  q+  * ? qq   * ≈M 2 e -E/T

12. Electron pair measurement in PHENIX 2 central arms: electrons, photons, hadrons –charmonium J/ ,  ’ -> e + e - –vector meson r, w,  -> e + e - –high p T p o, p +, p - –direct photons –open charm –hadron physics Au-Au & p-p spin PC1 PC3 DC magnetic field & tracking detectors e+e+ ee   designed to measure rare probes: + high rate capability & granularity + good mass resolution and particle ID - limited acceptance

13. LMR-I = quasi-real virtual photon o LMR I (p T >> m ee ) 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 p T slices p+p in agreement with cocktail Au+Au low mass enhancement concentrated at low p T p+p Au+Au arXiv:0912.0244

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

16. Virtual Photon Measurement  Case of hadrons (  0,  ) (Kroll-Wada) S = 0 at M ee > M hadron  Case of direct  * If p T 2 >>M ee 2 S = 1  For m>m ,  0 background (~80% of background) is removed  S/B is improved by a factor of five Any source of real  can emit  * with very low mass. Relation between the  * yield and real photon yield is known. Process dependent factor 00  Direct 

17. Determination of  * fraction, r r = direct  * /inclusive  * Direct  * /inclusive  * is determined by fitting the following function f direct : direct photon shape with S = 1. arXiv:0804.4168 arXiv:0912.0244 Fit in 120-300MeV/c 2 (insensitive to  0 yield) The mass spectrum follows the expectation for m > 300 MeV  S(m) ~ 1

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

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

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

21. Summary of the fit Significant yield of the exponential component (excess over the scaled p+p) The inverse slope T AuAu = 221±19±19 MeV (>T c ~ 170 MeV) –p+p fit funciton: A pp (1+p t 2 /b) -n –If power-law fit is used for the p+p spectrum, T AuAu = 240±21 MeV

22. Theory comparison Hydrodynamical models are compared with the data D.d’Enterria &D.Peressounko T=590MeV,  0 =0.15fm/c S. Rasanen et al. T=580MeV,  0 =0.17fm/c D. K. Srivastava T=450-600MeV,  0 =0.2fm/c S. Turbide et al. T=370MeV,  0 =0.33fm/c J. Alam et al. T=300MeV,  0 =0.5fm/c F.M. Liu et al. T=370MeV,  0 =0.6 fm/c Hydrodynamical models are in qualitative agreement with the data

23. Initial temperature From data: T ini > Tave = 220 MeV From models: T ini = 300 to 600 MeV  0 = 0.15 to 0.6 fm/c Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV T C from Lattice QCD ~ 170 MeV T ave (fit) = 221 MeV

24. Summary and conclusion We have measured e+e- pairs for m<300MeV and 1<p T <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 T AA scaled pp is exponential, with inverse slope T=221 ±19(stat)±19(sys) MeV Hydrodynamical models with T init =300-600MeV at  0 =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