Direct Photons in Au+Au Collisions Measured with the PHENIX Detector at RHIC Richard Petti For the PHENIX Collaboration Department of Physics and Astronomy Stony Brook University WWND 2011 R. Petti WWND11

Physics Motivation Direct photons are an important probe of the QGP Do not interact strongly with the QGP Emitted at all stages of the fireball evolution Rich physics at all momenta High momentum is dominated by hard processes Modification of the fragmentation function using γ jets A measured v2 could indicate pre-thermalized flow Low momentum dominated by thermal radiation Tells us something about the temperature of the plasma v2 is very sensitive to thermalization time, τ0 Possibly constrain this with data R. Petti WWND11

Further Physics Motivation Chatterjee, Srivastava PhysRevC79, 021901,’09 A little bit of theory Assume that beyond some τ0 the system is thermalized Ideal hydro evolution The τ0 is varied, while keeping the rapidity density constant Main points of the paper v2 of hadrons insensitive to τ0 v2 of thermal photons is Small τ0 → larger initial temp. → larger QGP contrib. → smaller v2 R. Petti WWND11

The PHENIX Detector at RHIC and Techniques for Photon Measurment Measure photons that directly deposit energy into the EMCal Works best at higher momentum Measure virtual photons that internally convert into e+e- pairs Allows a clean low pT measurement Yield of virtual photons is related to real photon production Measure real photons that externally convert in material into e+e- pairs Complementary to virtual photon method R. Petti WWND11

Measuring Photons in the EMCal Direct photon pT spectrum Little/no suppression of photons in central Au+Au Direct photon v2 R. Petti WWND11

Measuring Virtual Photons (I) Processes which produce photons can also produce virtual photons Subsequently decay into low-mass e+e- pairs Electrons can be reconstructed cleanly at low pT The relation between photon and pair production can be written as R. Petti WWND11

Measuring Virtual Photons (II) Low 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 e- 0 e+  R. Petti WWND11

Compare Mass Distribution to Cocktail R. Petti WWND11 Phys. Rev. Lett. 104, 132301 (2010)

Extracting the Fraction of Direct Photons Enhancement Fit mass distribution with a two-component function f(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 R. Petti WWND11 Phys. Rev. Lett. 104, 132301 (2010)

Low Momentum Direct Photons from the Virtual Photon Method PHENIX has measured low pT direct photon ratio in various collision systems, showing clear enhancement in A-A No significant enhancement R. Petti WWND11 See talk by D. Watanabe, Fri. Feb. 11, 9:00

Measuring External Photon Conversions: Identification PHENIX tracks outside magnetic field Need to assume the origin Assume particles come from event vertex We are interested in conversions in the HBD backplane (radius ≈ 60cm) Now our assumption is incorrect Gives pairs an artificial opening angle Leads to an apparent mass R. Petti WWND11

Some Simulations of Conversions Full GEANT simulation of photon conversions Mass = 12MeV

Some Simulations of Conversions Full GEANT simulation of photon conversions Assume all particles come from a radius of 60cm (ATM)

External Conversions – Applying the Cuts Conversions at the HBD backplane should have no track separation here Black – all e+/e- pairs Red – cut on small dphi of the pair Violet – cut on small dphi and dz at HBD shell A.U. A.U. Alternate Track Model Reco Normal Reco R. Petti WWND11

Towards a Direct Photon v2 Using External Conversions We want to know whether direct photons flow We need three ingredients to calculate the direct photon v2 Need the inclusive photon v2 Need the fraction of direct photons above decay photons (Rγ = Ninc/Nhadron = r+1) Need the π0 v2 (since most of the inclusive photons are from π0 decays) R. Petti WWND11

Inclusive Photon v2 with External Conversions 0.2 0.1 0.2 0.1 0.2 0.1 0 1 2 3 0 1 2 3 0 1 2 3 photon pT [GeV/c] photon pT [GeV/c] photon pT [GeV/c] Correlate particles to the reaction plane angle Average of two methods R. Petti WWND11

Inclusive Photon v2 with External Conversions 0.2 0.1 0.2 0.1 0.2 0.1 0 1 2 3 0 1 2 3 0 1 2 3 photon pT [GeV/c] photon pT [GeV/c] photon pT [GeV/c] Compare to Phys. Rev. Lett. 96, 032302 (2006) in the open symbols R. Petti WWND11

Getting at the Direct Photon Fraction Developed a π0 tagging technique to measure photons that come from π0 decays γ π0 R. Petti WWND11

Measuring Rγ - The Double Ratio e+/e- Pair efficiency e+/e- Pair acceptance DATA SIMULATION Conversion factor f: Conditional Acceptance of second decay photon Photon efficiency Pair acceptance and efficiency cancels in the ratio Conversion factor cancels R. Petti WWND11

Momentum Spectra Nγincl Nγπ0tag π0 Reconstruction Arbitrary units

Summary At low momentum, direct photons can tell us about Temperature of the plasma Thermalization time Looking at dielectron pairs is a nice tool to get a clean direct photon signal at low pT PHENIX 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+A Progress being made on using the external photon technique to measure direct photon v2 R. Petti WWND11

Backups R. Petti WWND11

Purity of the Photon Sample During 2007, half the HBD was removed for repair mid-run We can analyze the arms separately, with the HBDless arm representing the background The background observed should be the same in both arms Indicates that background pairs contribute less than 3% of the total photon yield R. Petti WWND11

Conversion Recalibrator in 2D HBD conversions Dalitz R. Petti WWND11

Elliptic Flow (v2) In non-central collisions Spatial anisotropy → pt anisotropy Distribution of particles in phi is measured and Fourier decomposed v2 is a measure of the elliptic flow strength R. Petti WWND11