Neutral pion production with respect to centrality and reaction plane in Au+Au collisions at sqrt(s_NN)=200 GeV

TL;DR

This study measures neutral pion production in gold-gold collisions at 200 GeV, revealing a small rise in suppression with transverse momentum and a strong azimuthal dependence related to the collision geometry, challenging some theoretical models.

Contribution

It provides high-precision data on neutral pion suppression and azimuthal dependence, testing and challenging existing parton energy loss models in heavy-ion collisions.

Findings

Neutral pion suppression factor is about 5.

A small but significant increase in R_AA with p_T in central collisions.

Strong azimuthal dependence of R_AA observed.

Abstract

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+/-0.01(stat)+/-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.