Measurements of low pT direct photons in PHENIXYorito Yamaguchi for the PHENIX collaborationCNS, University of Tokyo04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Quark Gluon Plasma De-confined phase of quarks and gluons Experimental approach at RHIC sNN = 200GeV Au+Au collisions Direct photons are an important probe to investigate the characteristics of evolution of the matter created by heavy ion collisions. Penetrate the strong interacting matter Emitted from every stage of collisions Hard photons (High pT) Initial hard scattering, Pre-equilibrium Thermal photons (Low pT) Carry the thermodynamic information from QGP and hadron gasIntroduction04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Photon Measurement in PHENIX Hard photon Strong suppression of high pT hadrons helps to improve the S/N ratio Successfully measured for both p+p and Au+Au collisions Measured pT range up to 14GeV/c Good agreement with pQCD Thermal photon Thermal radiation from QGP Primary contributor in low pT range up to 3-6GeV/c Direct evidence of thermal equilibration Thermal photon measurement is very challenging because it is very hard due to a large background from hadron decays.04/11/2008WWND 2008 @ South Padre IslandAu+Au*/16

WWND 2008 @ South Padre Island

Low pT PhotonsLong-awaited results for both p+p and Au+Au Experimental determination is very important since applicability of pQCD is doubtable in low pT region.In real photon measurement Measured yield with a large systematic errorDifficulty on measuring low pT real direct photonsFinite energy resolution of the EMCalLarge hadron backgroundAdvantages on measuring virtual photonsHigh momentum resolution of the Drift ChamberReliable estimation of the hadron decay components using Kroll-Wada formulaAlternative method to measure low pT direct photons Measure e+e- pairs from virtual direct photons04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Virtual Photon Measurement Case of Hadrons

Obviously S = 0 at Mee > Mhadron Case of g* If pT2>>Mee2 Possible to separate hadron decay components from real signal in the proper mass window. Any source of real g can emit g* with very low mass. Convert direct g* fraction to real direct photon yieldKroll-Wada formulaS : Process dependent factor04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Signal ExtractionarXiv: 0706.3034arXiv: 0802.0050Au+Aup+p Real signal di-electron continuum Background sources Combinatorial background Material conversion pairs 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 jets04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Hadronic Cocktail CalculationarXiv: 0802.0050 Mass distributions from hadron decays are simulated by Monte Carlo. p0, h, h, w, f, r, J/y, y Effects on real data are implemented. PHENIX acceptance, detector effect, efficiencies Hadronic cocktail was well tuned to individually measured yields of mesons in PHENIX for both p+p and Au+Au collisions.04/11/2008WWND 2008 @ South Padre Island Remaining pairs after background subtraction Real signal + Hadron decay components Estimate hadron components using hadronic cocktail*/16

WWND 2008 @ South Padre Island

Cocktail ComparisonRencontres de Moriond - QCD and High Energy Interactions -03/14/2008arXiv: 0802.0050arXiv: 0706.3034Au+Aup+p p+p Excellent agreement with cocktail Au+Au Large enhancement in low mass region Integrated yield in150MeV < mee < 750MeV Real/cocktail = 3.4 0.2(stat) 1.3(sys) 0.7(model)*/14

Rencontres de Moriond - QCD and High Energy Interactions -

pT Sliced Mass Spectra Shape differences between p+p and Au+Au are larger at lower pT. Au+Au p+p04/11/2008WWND 2008 @ South Padre IslandNormalized by the yield in mee < 100MeV*/16

WWND 2008 @ South Padre Island

WWND 2008 @ South Padre Island04/11/2008pT DependenceAu+Au pT

Cocktail Comparisonp+p Good agreement between real and cocktail Small excess at higher pTAu+Au Good agreement in Mee < 50MeV Enhancement is clearly seen above 100MeV.1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Determination of g* fraction, rr : direct g*/inclusive g*Direct g*/inclusive g* is determined by fitting the following function for each pT bin. Fit in 80-300MeV gives Assuming direct g* shape c2/NDF=11.6/10 Assuming h shape instead of direct g* shape c2/NDF=21.1/10 Twice as much as measured h yield Assumption of direct g* is favorable.Mee (GeV/c2)Reminder : fdirect is given by Kroll-Wada formula with S = 1.04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

direct g*/inclusive g*Curves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang.p+p Consistent with NLO pQCD better agreement with small Au+Au Clear enhancement above NLO pQCDp+pAu+Au04/11/2008WWND 2008 @ South Padre IslandBase line*/16

WWND 2008 @ South Padre Island

Direct Photon Spectrap+p First measurement in 1-4GeV/c Consistent with NLO pQCD Serves as a crucial referenceThe virtual direct photon fraction is converted to the direct photon yield.Au+Au Above binary scaled NLO pQCD Excess comes from thermal photons?04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Theory ComparisonD.dEnterria, D.Peressounko, Eur.Phys.J.C 46 (2006)T0ave=360 MeV (T0max=590 MeV)t0=0.15 fm/c04/11/2008WWND 2008 @ South Padre IslandS.Turbide, R.Rap, C.Gale, Phys.Rev.C 69 (2004)T0max=370 MeV t0=0.33 fm/c*/16

WWND 2008 @ South Padre Island

Summary & Outlook Direct photon measurements with virtual photon method in p+p and Au+Au collisions have been done at RHIC-PHENIX. The fractions of direct g* to inclusive g* above pT of 1GeV/c are obtained by making a shape comparison between real pairs and a hadronic cocktail. This is the first time that direct photon production in p+p collisions has been measured in 1

backup04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

PHENIX DetectorMinimum Bias data sample (triggered by BBC) & Electron triggered data sample (p+p)BBCz-vertexDC, PC1Tracking RICH & EMCalElectron ID Electron Trigger04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Material Conversion Pair CutThe pairs from material conversion should be removed.These pairs can be recognized by its orientation relative to the magnetic field.No cutM

Additional Correlated BackgroundJet cross pairDalitz + conversion cross pairCorrelated + combinatorial background is very good agreement with the real like sign mass spectrum.Systematic error due to background subtraction 2%04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Combinatorial Background (Au+Au)Normalization factor is determined by like sign pairs.N++ and N-- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV)Normalization: 2N++ N-- Uncertainty due to statistics of N++ and N--: 0.12%

04/11/2008WWND 2008 @ South Padre Island*/16

WWND 2008 @ South Padre Island

Centrality DependenceWWND 2008 @ South Padre Island04/11/2008 Integrated yield divided by Npart/2150MeV

Theory Comparison 2Freeze-out Cocktail + random charm + r spectral functionLow massM>0.4GeV/c2: some calculations OKM