Centrality and transverse momentum dependencies of minijets and hadrons in Au-Au collisions

Abstract

In the study of hadron production in Au-Au collisions at RHIC minijets play an important role in generating shower partons in the intermediate \ppt\ region. Momentum degradation of the hard and semihard partons as they traverse the inhomogeneous medium at various azimuthal angles results in a complicated convolution of geometrical, nuclear and dynamical factors that cannot usually be described in a transparent way. In this work a compact formula is found that represents the inclusive distributions of minijets of any parton type at the surface of the medium for any collision centrality. They take into account the contributions from all initiating partons created at any point in the medium. By comparing with the case of no energy loss, a ratio has been determined that is analogous to the nuclear modification factor for minijets. Phenomenological reality of such distributions is examined by calculating the hadronization of the minijets in the recombination model. Good fits of the data on pion, kaon and proton production throughout the intermediate \ppt\ region have been obtained by adjusting the parameters controlling the magnitude of the thermal partons and the degradation rates of the semihard partons. The result gives support to the minijet spectra at any centrality on the one hand, and the hadronization procedure used on the other. An important property made manifest in this study is that quarks and gluons must not lose energy in the same way because the partons form mesons and baryons differently by recombination and the momenta of quarks and gluons must be degraded at different rates in order to reproduce the experimental pion and proton spectra. This is a feature that renders invalid the notion of parton-hadron duality or other hadronization schemes based on similar ideas.