| With substantial additional statistics due to the inclusion of a new Run2 trigger data sample, the PHENIX Collaboration has measured the first positive direct photon (gammadirect) signal in Au+Au collisions at sNN = 200 GeV and midrapidity (|y| ≤ 0.35). The measurement is made in 10 centrality bins covering 0--92% of the full geometric cross section. Additionally, the new data has extended the previous PHENIX pi 0 measurement [70] by 4 GeV/c in its pT range, matching the gammadirect measurement with a pT range of 1--14 GeV/c which make them the highest pT measurements yet in RHIC Au+Au. The gammadirect yields are compared amongst themselves, with references, and with the pi 0. The suppression in meson hard scattering previously discovered at RHIC [70] is found to be absent in the direct photons. Specifically, using the NLO perturbative QCD prediction of gammadirect as a reference, the nuclear modification factor RAA is found to be consistent with one and pT-independent for pT > 6 GeV/c.; Thus, like the d+Au results, these direct photon measurements represent possibly the best available confirmation of the conclusion that the aforementioned suppression effect is not due to differences in initial state hard-scattering, but rather, is due to a final state medium which quenches hard quarks and gluons, but not hard direct photons. This conclusion is consistent with final state modifications predicted as indicators of Quark Gluon Plasma (QGP) formation [172]. In addition, the possible model-generated mechanisms for this suppression are constrained further (though only slightly) by our updated pi0 RAA results due to the higher pT values reached. The suppression continues to be independent of pT, signaling strong energy dependence.; The direct photon invariant yields in (pT < 6 GeV/c) region are compared to predictions of thermal production and new mechanisms of photon enhancement [90],[179]. Large uncertainties do not allow any definite conclusions about observations or constraints of such enhancements. |
