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

TL;DR

This study measures neutral-pion suppression in gold-gold collisions across energies from 39 to 200 GeV, revealing energy-dependent suppression patterns and challenging existing theoretical models at lower energies.

Contribution

It provides new measurements of pi^0 spectra at multiple energies and compares suppression patterns, highlighting the limitations of current models at lower collision energies.

Findings

Significant suppression of pi^0 at high p_T in central collisions.

Energy dependence observed in nuclear modification factors R_AA.

Perturbative QCD models fail to describe 39 GeV data.

Abstract

Neutral-pion, pi^0, spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at sqrt(s_NN) = 39 and 62.4 GeV and compared to earlier measurements at 200 GeV in the 1<p_T<10 GeV/c transverse-momentum (p_T) range. 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 p+p-collision spectra. The nuclear-modification factors (R_AA) show significant suppression and a distinct energy dependence at moderate p_T in central collisions. At high p_T, R_AA is similar for 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 the relative importance of initial-state effects and soft processes increases at lower energies. A conclusion that the region where hard processes are dominant is reached only at higher p_T, is also supported by the x_T dependence of the x_T-scaling power-law exponent.