Studying the sources of high p T direct photons in Au+Au collisions at RHIC/PHENIX

1. Penetrating probes emitted at all stages then surviving unscathed ( ae << as) “Historians” of the heavy ion collision: encode all sub-processes at all times But for the very same reason their message is hard to decipher! RAA with pQCD Cartoon only: sources of g, mean pT vs time (dashed: hadrons) hard scatt pT (GeV) jet Brems. Fragmentation: non-isolated Bremsstrahlung: non-isolated Jet-photon conversion: isolated “Primordial”: isolated jet-thermal Note: assuming no energy loss  fragmentation g is isotropic  jet-g conversion dominates v2 v2<0 jet fragmentation RAA with p+p data sQGP hadron gas hadron decays 1/ Get the NN part (including isospin effect) 2/ Get the jet-conversion (jet-th) part from isolated, v2<0 3/ Get the fragmentation from non-isolated, v2>0 4/ …  TALL ORDER, TO SAY THE LEAST So if something like this were the truth, in principle you could try to disentangle the components like this: All we can see is the projection to this axis with the dashed sources as background dominated up to medium pT by this log t PHENIX Preliminary sNN=200GeV Au+Au 1 10 107 (fm/c) W. Vogelsang, NLO pQCD + isospin jet fragment photon v2 > 0 annihilation compton scattering jet v2 > 0 Medium induced (inc.energy loss) v2 < 0 Studying the sources of high pT direct photons in Au+Au collisions at RHIC/PHENIX G. David Brookhaven National Laboratory QM’08, Jaipur, India The promise of (and the problem with) direct photons: What this translates to in a particular framework: (C. Gale, Nucl. Phys. A785 (2007) 93 But even this part has at least two components: from gluon Compton-scattering and fragmentation A possible way to decompose Testable in p+p (and in the future maybe also in Au+Au): prompt photons are isolated, fragmentation photons are not Is this (measuring photon flow) realistic? Recall previous results: Improvement at medium and high pT : better reaction plane, higher statistics This is very important, but cannot necessarily be translated directly to AuAu (isospin effect) How about “thermal” photons? New results on direct photons with “internal conversion” Also, the question can be asked: do structure functions change? So far the response (from Run-3 d+Au) is negative, but Run-8 might change that (high pT reach!) Translated to a (thermal?) direct photon spectrum using inclusive yields from calorimetry These two plots were (physically) cut out of my poster because PPG086 was not released (happened at the coll. mtg, after the poster has already been printed) Then what is known so far about high pT photons in heavy ions? Pions are (luckily) strongly suppressed, but there is an apparent suppression of direct photons as well at high pT if the nuclear modification factor is calculated with p+p (isospin effect!) Brand new results in Cu+Cu do not exhibit similar suppression at high pT If xT scaling holds, at 62 GeV we should see the same photon suppression at ~pT=7-8GeV/c Integrated RAA for photons and pions, for the two different systems Next step (this is a strictly technical plot!): trying to understand the high pT inclusive yields. Basic problems: loss/mis-identification of merging g-pairs from p0. But merging starts at different pT for decays with different asymmetry. However, after correction the yields extracted/corrected in various windows of asymmetry should become equal (acceptance is virtually constant at high pT). The four colors show corrected yields (and their ratio to the total) for 0.0-0.2 (black), 0.2-0.4 (red), 0.4-0.6 (green) and 0.6-0.8 (blue) asymmetry. This is Run-4, and dominated by statistical error – Run-7 data will allow us to establish the merging corrections with small errors from the data themselves (without relying on simulations)