Direct Photons in Au+Au Collisions Measured with the PHENIX Detector at RHIC

Richard PettiFor the PHENIX CollaborationDepartment of Physics and AstronomyStony Brook UniversityWWND 2011Direct Photons in Au+Au Collisions Measured with the PHENIX Detector at RHIC1R. Petti WWND111Physics MotivationDirect photons are an important probe of the QGPDo not interact strongly with the QGPEmitted at all stages of the fireball evolutionRich physics at all momentaHigh momentum is dominated by hard processesModification of the fragmentation function using jetsA measured v2 could indicate pre-thermalized flowLow momentum dominated by thermal radiationTells us something about the temperature of the plasmav2 is very sensitive to thermalization time, 0Possibly constrain this with data2R. Petti WWND112

Further Physics MotivationR. Petti WWND113A little bit of theoryAssume that beyond some 0 the system is thermalizedIdeal hydro evolutionThe 0 is varied, while keeping the rapidity density constantMain points of the paperv2 of hadrons insensitive to 0v2 of thermal photons is

Chatterjee, Srivastava PhysRevC79, 021901,09Small 0 larger initial temp. larger QGP contrib. smaller v2 3The PHENIX Detector at RHIC and Techniques for Photon MeasurmentR. Petti WWND114Measure photons that directly deposit energy into the EMCalWorks best at higher momentumMeasure virtual photons that internally convert into e+e- pairsAllows a clean low pT measurementYield of virtual photons is related to real photon productionMeasure real photons that externally convert in material into e+e- pairsComplementary to virtual photon method

4Measuring Photons in the EMCalR. Petti WWND115

Little/no suppression of photons in central Au+AuDirect photon pT spectrum

Direct photon v25Measuring Virtual Photons (I)R. Petti WWND116Processes which produce photons can also produce virtual photonsSubsequently decay into low-mass e+e- pairsElectrons can be reconstructed cleanly at low pTThe relation between photon and pair production can be written as

6Low mass pairs are made (mass < 0.3 GeV/c2)Pairs from external photon conversions are removed

Correlated sources of background (cross pairs) are removed by studying like-sign pairs

A cocktail of hadronic decays is calculated with MC and is compared with the data

Measuring Virtual Photons (II)R. Petti WWND117

e-0e+e-e+7Compare Mass Distribution to CocktailR. Petti WWND118

Phys. Rev. Lett. 104, 132301 (2010)8Extracting the Fraction of Direct PhotonsR. Petti WWND119

EnhancementPhys. Rev. Lett. 104, 132301 (2010)Fit mass distribution with a two-component functionf(mee) = (1 r)fc(mee) + rfdir(mee)Convert fraction r to direct photon yield as dNdir(pT) = r x dNincl(pT)dNincl(pT) is determined from the yield of e+e- pairs using 9Low Momentum Direct Photons from the Virtual Photon MethodR. Petti WWND1110PHENIX has measured low pT direct photon ratio in various collision systems, showing clear enhancement in A-A

See talk by D. Watanabe, Fri. Feb. 11, 9:00No significant enhancement10Measuring External Photon Conversions: IdentificationR. Petti WWND1111PHENIX tracks outside magnetic fieldNeed to assume the originAssume particles come from event vertexWe are interested in conversions in the HBD backplane (radius 60cm)Now our assumption is incorrectGives pairs an artificial opening angleLeads to an apparent mass

11Some Simulations of ConversionsFull GEANT simulation of photon conversions12

Mass = 12MeV

Some Simulations of ConversionsFull GEANT simulation of photon conversionsAssume all particles come from a radius of 60cm (ATM)13

External Conversions Applying the CutsR. Petti WWND1114Black all e+/e- pairsRed cut on small dphi of the pairViolet cut on small dphi and dz at HBD shell Normal RecoAlternate Track Model RecoConversions at the HBD backplane should have no track separation hereA.U.A.U.14Towards a Direct Photon v2 Using External ConversionsR. Petti WWND1115We want to know whether direct photons flowWe need three ingredients to calculate the direct photon v2Need the inclusive photon v2Need the fraction of direct photons above decay photons (R = Ninc/Nhadron = r+1)Need the 0 v2 (since most of the inclusive photons are from 0 decays)

15R. Petti WWND11

Inclusive Photon v2 with External Conversions16Correlate particles to the reaction plane angleAverage of two methodsv2v2v20.2

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photon pT [GeV/c]0 1 2 3 photon pT [GeV/c]0 1 2 3 photon pT [GeV/c]

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R. Petti WWND11Inclusive Photon v2 with External Conversions17Compare to Phys. Rev. Lett. 96, 032302 (2006) in the open symbolsv2v2v20.2

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photon pT [GeV/c]0 1 2 3 photon pT [GeV/c]0 1 2 3 photon pT [GeV/c]17Getting at the Direct Photon FractionR. Petti WWND1118Developed a 0 tagging technique to measure photons that come from 0 decays

018Measuring R - The Double RatioR. Petti WWND1119Pair acceptance and efficiency cancels in the ratioConversion factor cancels

DATAe+/e- Pair efficiencye+/e- Pair acceptancePhoton efficiencyf: Conditional Acceptance of second decay photonSIMULATION

Conversion factor19

Momentum Spectra20

N0tag0 Reconstruction

NinclArbitrary unitsArbitrary unitsSummaryR. Petti WWND1121At low momentum, direct photons can tell us aboutTemperature of the plasmaThermalization timeLooking at dielectron pairs is a nice tool to get a clean direct photon signal at low pTPHENIX is measuring direct photons in various collisional systems (including baseline p+p and d+Au)No significant enhancement in the baseline systems, but significant enhancement in A+AProgress being made on using the external photon technique to measure direct photon v221BackupsR. Petti WWND112222Purity of the Photon SampleR. Petti WWND1123During 2007, half the HBD was removed for repair mid-runWe can analyze the arms separately, with the HBDless arm representing the backgroundThe background observed should be the same in both armsIndicates that background pairs contribute less than 3% of the total photon yield

23Conversion Recalibrator in 2DR. Petti WWND1124

HBD conversionsDalitz24In non-central collisionsSpatial anisotropy pt anisotropyDistribution of particles in phi is measured and Fourier decomposedv2 is a measure of the elliptic flow strengthElliptic Flow (v2)R. Petti WWND1125

Non-central collision25