Neutral pion production with respect to centrality and reaction plane in Au+Au collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV

Neutral pion production with respect to centrality and reaction plane in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV

A. Adare et al. (PHENIX Collaboration)

Phys. Rev. C 87, 034911 – Published 28 March 2013

Abstract

The PHENIX experiment has measured the production of $π^{0}$ s 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 $η / π^{0}$ 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. $π^{0}$ are 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 $\pm_{0.0029}^{0.0034}$ (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 $π^{0}$ yield is measured at different angles with respect to the event plane; a strong azimuthal dependence of the $π^{0}$ $R_{AA}$ 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 perturbative quantum chromodynamics are insufficient to describe the data, while a hybrid model utilizing pQCD for the hard interactions and anti-de-Sitter space/conformal field theory (AdS/CFT) for the soft interactions is consistent with the data.