Abstract
Neutral-pion π0 spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at √sNN=39 and 62.4 GeV and compared with earlier measurements at 200 GeV in a transverse-momentum range of 1<pT<10 GeV/c. The high-pT 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 (RAA) show significant suppression, with a distinct energy, centrality, and pT dependence. Above pT=7 GeV/c, RAA is similar for √sNN=62.4 and 200 GeV at all centralities. Perturbative-quantum-chromodynamics calculations that describe RAA well at 200 GeV fail to describe the 39 GeV data, raising the possibility that, for the same pT region, the relative importance of initial-state effects and soft processes increases at lower energies. The pT range where π0 spectra in central Au+Au collisions have the same power as in p+p collisions is ≈5 and 7 GeV/c for √sNN=200 and 62.4 GeV, respectively. For the √sNN=39 GeV data, it is not clear whether such a region is reached, and the xT dependence of the xT-scaling power-law exponent is very different from that observed in the √sNN=62 and 200 GeV data, providing further evidence that initial-state effects and soft processes mask the in-medium suppression of hard-scattered partons to higher pT as the collision energy decreases.
