Detailed study of high-p_T neutral pion suppression and azimuthal anisotropy in Au+Au collisions at √s_NN=200 GeV

Abstract

Measurements of neutral pion (π⁰) production at midrapidity in √s_NN=200 GeV Au+Au collisions as a function of transverse momentum, p_T, collision centrality, and angle with respect to reaction plane are presented. The data represent the final π⁰ results from the PHENIX experiment for the first RHIC Au+Au run at design center-of-mass energy. They include additional data obtained using the PHENIX Level-2 trigger with more than a factor of 3 increase in statistics over previously published results for p_T>6 GeV/c. We evaluate the suppression in the yield of high-p_T π^0's relative to pointlike scaling expectations using the nuclear modification factor R_AA. We present the p_T dependence of R_AA for nine bins in collision centrality. We separately integrate R_AA over larger p_T bins to show more precisely the centrality dependence of the high-p_T suppression. We then evaluate the dependence of the high-p_T suppression on the emission angle Δφ of the pions with respect to event reaction plane for seven bins in collision centrality. We show that the yields of high-p_T π^0's vary strongly with Δφ, consistent with prior measurements [1, 2]. We show that this variation persists in the most peripheral bin accessible in this analysis. For the peripheral bins we observe no suppression for neutral pions produced aligned with the reaction plane, whereas the yield of π^0' produced perpendicular to the reaction plane is suppressed by a factor of ~2. We analyze the combined centrality and Δφ dependence of the π⁰ suppression in different p_T bins using different possible descriptions of parton energy loss dependence on jet path-length averages to determine whether a single geometric picture can explain the observed suppression pattern.