Evolution ofπ0Suppression inAu+AuCollisions fromsNN=39to 200 GeV

Neutral-pion ${\ensuremath{\pi}}^{0}$ spectra were measured at midrapidity ($|y|l0.35$) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}=39$ and 62.4 GeV and compared with earlier measurements at 200 GeV in a transverse-momentum range of $1l{p}_{T}l10\text{ }\text{ }\mathrm{GeV}/c$. 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 ${p}_{T}=7\text{ }\text{ }\mathrm{GeV}/c$, ${R}_{AA}$ is similar for $\sqrt{{s}_{\mathrm{NN}}}=62.4$ and 200 GeV at all centralities. Perturbative-quantum-chromodynamics calculations that describe ${R}_{AA}$ well at 200 GeV fail to describe the 39 GeV 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 ${\ensuremath{\pi}}^{0}$ spectra in central $\mathrm{Au}+\mathrm{Au}$ collisions have the same power as in $p+p$ collisions is $\ensuremath{\approx}5$ and $7\text{ }\text{ }\mathrm{GeV}/c$ for $\sqrt{{s}_{\mathrm{NN}}}=200$ and 62.4 GeV, respectively. For the $\sqrt{{s}_{\mathrm{NN}}}=39\text{ }\text{ }\mathrm{GeV}$ 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 $\sqrt{{s}_{\mathrm{NN}}}=62$ 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 ${p}_{T}$ as the collision energy decreases.