Neutral pion production with respect to centrality and reaction plane in Au$+$Au collisions at $\sqrt{s_{NN}}$=200 GeV
Phys.Rev.C 87 (2013) 034911, 2013.
The collaboration

Abstract (data abstract)
BNL-RHIC. The PHENIX experiment has measured the production of neutral pions in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV. The new data offer a fourfold increase in recorded luminosity, providing higher precision and a larger reach in transverse momentum, $p_T$, to 20 GeV/c. The production ratio of $\eta/\pi^0$ is 0.46$\pm$0.01(stat)$\pm$0.05(syst), constant with $p_T$ and collision centrality. The observed ratio is consistent with earlier measurements, as well as with the p+p and d+Au values. The production of $\pi^0$ is suppressed by a factor of 5, as in earlier findings. However, with the improved statistical precision a small but significant rise of the nuclear modification factor, $R_{AA}$, vs $p_T$, with a slope of 0.0106$^(+0.0034)_(-0.0029)[GeV/c]^-1$, is discernible in central collisions. A phenomenological extraction of the average fractional parton energy loss shows a decrease with increasing $p_T$. To study the path length dependence of suppression, the $\pi^0$ yield was measured at different angles with respect to the event plane; a strong azimuthal dependence of the $\pi^0 R_{AA}$ is observed. The data are compared to theoretical models of parton energy loss as a function of the path length, L, in the medium. Models based on pQCD are insufficient to describe the data, while a hybrid model utilizing pQCD for the hard interactions and AdS/CFT for the soft interactions is consistent with the data.

Version 2 modifications: Revised abstract to remove LaTeC math inconsitency