Abstract
The PHENIX experiment has measured the production of π0s in Au+Au collisions at √sNN = 200 GeV. The new data offer a fourfold increase in recorded luminosity, providing higher precision and a larger reach in transverse momentum, pT, to 20 GeV/c. The production ratio of η/π0 is 0.46±0.01(stat)±0.05(syst), constant with pT and collision centrality. The observed ratio is consistent with earlier measurements, as well as with the p+p and d+Au values. π0 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 RAA vs pT, with a slope of 0.0106±0.00340.0029 (Gev/c)−1, is discernible in central collisions. A phenomenological extraction of the average fractional parton energy loss shows a decrease with increasing pT. To study the path-length dependence of suppression, the π0 yield is measured at different angles with respect to the event plane; a strong azimuthal dependence of the π0 RAA 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.
