Yorito Yamaguchi For the PHENIX collaboration CNS, University of Tokyo 10/14/2008ATHIC2008 1/13

Direct photons 10/14/2008ATHIC2008  Direct photon is an important probe to investigate the characteristics of evolution of the matter created by heavy ion collisions. Defined as photons from parton-parton interaction, NOT from hadron decays Emitted from every stage of collisions Penetrate the strong interacting matter Classified into 2 groups depending on their origin  Hard photons (High pT) – Initial hard scattering – Pre-equilibrium  Soft photons (Low pT) – Thermal radiation from QGP and Hadron gas hard: thermal: Decay photons (  0 → ,  → , …) 2/13

Thermal photons 10/14/2008ATHIC2008 3/13  Thermal radiation from QGP Primary contributor in 1.0 < pT < 3.0GeV/c Direct evidence of the QGP formation Carry the thermodynamic information of QGP ‒ Temperature, Degree of freedom and so on ‒ Approximately exponential function QGP Large background from hadron decays Applicability of pQCD is doubtable in low pT region. → Experimental determination is very important. S.Turbide et al PRC

q  g q e+e+ e-e- Thermal photon search in PHENIX 10/14/2008ATHIC2008 In ‘real’ photon measurement  Measured yield with a large systematic error Difficulty on measuring low pT “real” direct photons 1.Finite energy resolution of the EMCal 2.Large hadron background Advantages on measuring ‘virtual’ photons 1.High momentum resolution of the Drift Chamber 2.Reliable estimation of the hadron decay components using Kroll-Wada formula Alternative method to measure low pT direct photons → Measure e + e - pairs from ‘virtual’ direct photons 4/13

Virtual photon method 10/14/2008ATHIC2008  Dalitz decays of  0,  Obviously S = 0 at M ee > M hadron   * decay – If pT 2 >>M ee 2  Possible to separate hadron decay components from real signal in the proper mass window.  Any source of real  can emit  * with very low mass.  Convert direct  * fraction to real direct photon yield Kroll-Wada formula S : Process dependent factor 5/13

arXiv: arXiv: Au+Au p+p  Real signal di-electron continuum  Background sources 1. Combinatorial background 2. Material conversion pairs 3. Additional correlated background – Visible in p+p collisions – Cross pairs from decays with 4 electrons in the final state – Pairs in same jet or back-to-back jet After subtraction  Remaining pairs Real signal + Hadron decay components – Estimate hadron components using hadronic cocktail – Tuned to individually measured yields of mesons at PHENIX 10/14/2008ATHIC2008 6/13 Signal Extraction

pT sliced mass spectra 10/14/2008ATHIC < p T < 8 GeV/c0 < p T < 0.7 GeV/c 0.7 < p T < 1.5 GeV/c1.5 < p T < 8 GeV/c PHENIX Preliminary ○ Au+Au ● p+p  Possible sources of low mass enhancement virtual direct photons, qqbar and  annihilations  pT>1GeV/c && m ee <300MeV Small expected contributions from qqbar and  annihilations Satisfy pT 2 >>m ee 2 7/13

p+pAu+Au (MB) Low mass & high pT region 10/14/2008ATHIC2008 p+p  Good agreement between real and cocktail  Small excess at higher pT Au+Au  Good agreement in M ee < 50MeV  Enhancement is clearly seen above 100MeV. 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV arXiv: /13

Determination of  * fraction 10/14/2008ATHIC2008 r : direct  * /inclusive  * Direct  * /inclusive  * is determined by fitting the following function to the data for each pT bin.  Fit in MeV gives – Assuming direct  * shape  2 /DOF=13.8/10 – Assuming  shape instead of direct  * shape  2 /DOF=21.1/10 Twice as much as measured  yield → Assumption of direct  * is favorable. M ee (GeV/c 2 ) Reminder : f direct is given by Kroll-Wada formula with S = 1. arXiv: /13

direct  * /inclusive  * 10/14/2008ATHIC2008 Curves : NLO pQCD calculations with different theoretical scales. p+p  Consistent with NLO pQCD – better agreement with small µ Au+Au  Clear enhancement above NLO pQCD p+pAu+Au Base line L.E.Gordon, W.Vogelsang, PRD and W.Vogelsang, private communication arXiv: /13

Direct photon spectra 10/14/2008 p+p  First measurement in 1-3GeV/c  Consistent with NLO pQCD → Serves as a crucial reference Au+Au  Significant excess over binary- scaled p+p result for pT<3GeV/c. → Inverse slope of exponential fit is related to temperature of matter Exponential part Binary-scaled p+p result Inverse slope ATHIC2008 arXiv: /13

Theory comparison 10/14/2008ATHIC2008  Initial temperatures and times from theoretical models T=590MeV,  0 =0.15fm/c (D.d’Enterria et al.) T=580MeV,  0 =0.17fm/c (S.Rasanen et al.) T= MeV,  0 =0.2fm/c (D.K.Srivastava) T=370MeV,  0 =0.33fm/c (S.Turbide et al.) T=300MeV,  0 =0.5fm/c (J.Alam et al.) T=370MeV,  0 =0.6fm/c (F.Liu et al.)  All hydrodynamical models are agreement with the data within a factor of 2 in spite of their wide-ranging conditions. Thery compilation by D. d’Enterria and D. Peressounko EPJC46, 451 (2006)  Initial temperatures and times from theoretical models T=590MeV,  0 =0.15fm/c (D.d’Enterria et al.) T=580MeV,  0 =0.17fm/c (S.Rasanen et al.) T= MeV,  0 =0.2fm/c (D.K.Srivastava) T=370MeV,  0 =0.33fm/c (S.Turbide et al.) T=300MeV,  0 =0.5fm/c (J.Alam et al.) T=370MeV,  0 =0.6fm/c (F.Liu et al.)  All hydrodynamical models are in agreement with the data within a factor of 2 in spite of their wide-ranging conditions.  Agreement with real data for different centralities and a wide pT range. arXiv: /13

Summary and outlooks 10/14/2008ATHIC2008  Direct photon measurements with virtual photon method in p+p and Au+Au collisions have been done at RHIC-PHENIX.  Significant excess of direct photon yield above binary scaled p+p result is observed in Au+Au collisions. Fitting with exponential + binary-scaled p+p result provides inverse slope of 221±23±18MeV in central Au+Au collisions.  Theoretical models are in agreement with the data within a factor of 2 in spite of their wide-ranging initial conditions.  Same analysis in d+Au collisions is needed to evaluate contributions from nuclear effects to direct photon yield in pT < 3GeV/c. Cronin effect PDF of Au nucleus  p+p analysis with more statistics is now ongoing. Result with higher quality will be provided. pT region will be extended upward. 13/13