The evolution of minimum-bias parton fragmentation in nuclear collisions

Abstract

Hard components of $p_t$ spectra can be identified with minimum-bias parton fragmentation in nuclear collisions. Minimum-bias fragment distributions (FDs) can be calculated by folding a power-law parton energy spectrum with parametrized fragmentation functions (FFs) derived from $e^+$-$e^-$ and p-\=p collisions. Alterations to FFs due to parton "energy loss" or "medium modification" in Au-Au collisions are modeled by adjusting FF parametrizations consistent with rescaling QCD splitting functions. The parton spectrum is constrained by comparison with a p-p $p_t$ spectrum hard component. The reference for all nuclear collisions is the FD derived from in-vacuum $e^+$-$e^-$ FFs. Relative to that reference the hard component for p-p and peripheral Au-Au collisions is found to be {\em strongly suppressed} for smaller fragment momenta. At a specific point on centrality the Au-Au hard component transitions to enhancement at smaller momenta and suppression at larger momenta, consistent with FDs derived from medium-modified $e^+$-$e^-$ FFs.