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Physical Review Letters


Neutral pion transverse momentum (pT) spectra at midrapidity (jyj & 0:35) were measured in Cu þ Cu collisions at ffiffiffiffiffiffiffiffi sNN p ¼ 22:4, 62.4, and 200 GeV. Relative to 0 yields in p þ p collisions scaled by the number of inelastic nucleon-nucleon collisions (Ncoll) the 0 yields for pT * 2 GeV=c in central Cu þ Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu þ Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.



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