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Neutral-pion, pi^0, spectra were measured at midrapidity (|y|<0.35) in Au+Aucollisions at sqrt(s_NN) = 39 and 62.4 GeV and compared to earlier measurementsat 200 GeV in the 1<p_T<10 GeV/c transverse-momentum (p_T) range. The high-p_Ttail is well described by a power law in all cases and the powers decreasesignificantly with decreasing center-of-mass energy. The change of powers isvery similar to that observed in the corresponding p+p-collision spectra. Thenuclear-modification factors (R_AA) show significant suppression and a distinctenergy dependence at moderate p_T in central collisions. At high p_T, R_AA issimilar for 62.4 and 200 GeV at all centralities.Perturbative-quantum-chromodynamics calculations that describe R_AA well at 200GeV, fail to describe the 39 GeV data, raising the possibility that therelative importance of initial-state effects and soft processes increases atlower energies. A conclusion that the region where hard processes are dominantis reached only at higher p_T, is also supported by the x_T dependence of thex_T-scaling power-law exponent.

Evolution of pi^0 suppression in Au+Au collisions from sqrt(s_NN) = 39 to 200 GeV