High pT Azimuthal Anisotropy in Au+Au Collisions at √sNN = 200 GeV
Abstract
A noval state of nuclear matter, in which quarks and gluons are deconfined, yet strongly coupled to each other, is created in Au+Au collisions at the Relativisitic Heavy Ion Collider (RHIC). This matter, which displays strong collective flow, and large opacity to the fast moving partons, are commonly refered to as the strongly coupled quark gluon plasma (sQGP). Many efforts are ongoing to understand the microscopic properties of the strong interaction and the relaxation process leading to the rapid thermalization. The PHENIX experiment has measured the azimuthal anisotropy of &pi<super>0</super> at mid-rapidity (|Η| < 0.35) in Au+Au collisions at √s<sub>NN</sub> = 200 GeV in RHIC 2007 run (Run-7). It allows for detailed study of the anisotropy as a function of collision centrality and transverse momentum pT in the range of 1-18 GeV/c. The observed anisotropy shows a gradual decrease up to pT of 7-10 GeV/c and remains significantly above zero at pT > 10 GeV/c. The φ dependent nuclear modification factors show a large split between the in-plane and out-of-plane directions, a large difference which exceeds the expectation from the energy loss models. In addition, the anisotropy of &pi<super>0</super> at mid-rapidity are measured with respect to the reaction planes reconstructed by four different reaction plane detectors located at forward region (|Η| > 1.0), for the first time that the detailed study of the influence of non-flow due to jets on the measured v2 at high pT are presented. A jet absorption model is employed to study the importance of initial geometry and path length on the jet energy loss. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and fluctuations is insufficient to account for this discrepancy. Calculations that implement a path length dependence steeper than what is implied by current pQCD energy-loss models show reasonable agreement with the data.