Evolution of minimum-bias parton fragmentation in nuclear collisions

Abstract

Minimum-bias fragment distributions (FDs) are calculated by folding a power-law parton energy spectrum with parametrized fragmentation functions (FFs) derived from $e^+$-$e^-$ and p-p collisions. Changes in FFs due to parton "energy loss" or "medium modification" are modeled by altering FF parametrizations consistent with rescaling QCD splitting functions. The common parton spectrum is constrained by comparison with a p-p $p_t$ spectrum hard component. In-vacuum and in-medium FDs are compared with spectrum hard components from 200 GeV Au-Au collisions for several centralities. The reference for all nuclear collisions is the FD derived from in-vacuum $e^+$-$e^-$ FFs. The hard component for p-p and peripheral Au-Au collisions is found to be {\em strongly suppressed} for smaller fragment momenta, consistent with the FD derived from in-vacuum p-p FFs. At a particular centrality the Au-Au hard component transitions to enhancement at smaller momenta and suppression at larger momenta, consistent with FDs derived from in-medium $e^+$-$e^-$ FFs. Fragmentation systematics suggest that QCD color connections change dramatically in more-central A-A collisions. Spectrum systematics are inconsistent with saturation-scale arguments and parton thermalization.