Evolution of π0 suppression in Au+Au collisions from √s NN=39 to 200 GeV

Abstract

Neutral-pion π0 spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at √s NN=39 and 62.4GeV and compared with earlier measurements at 200GeV in a transverse-momentum range of 1<p T<10GeV/c. The high-p T tail is well described by a power law in all cases, and the powers decrease significantly with decreasing center-of-mass energy. The change of powers is very similar to that observed in the corresponding spectra for p+p collisions. The nuclear modification factors (R AA) show significant suppression, with a distinct energy, centrality, and p T dependence. Above p T=7GeV/c, R AA is similar for √s NN=62.4 and 200GeV at all centralities. Perturbative-quantum-chromodynamics calculations that describe R AA well at 200GeV fail to describe the 39GeV data, raising the possibility that, for the same p T region, the relative importance of initial-state effects and soft processes increases at lower energies. The p T range where π0 spectra in central Au+Au collisions have the same power as in p+p collisions is 5 and 7GeV/c for √s NN=200 and 62.4GeV, respectively. For the √s NN=39GeV data, it is not clear whether such a region is reached, and the x T dependence of the x T-scaling power-law exponent is very different from that observed in the √s NN=62 and 200GeV data, providing further evidence that initial-state effects and soft processes mask the in-medium suppression of hard-scattered partons to higher p T as the collision energy decreases. © 2012 American Physical Society.