Excitation functions of hadronic observablesfrom SIS to RHIC energies

Abstract

We calculate excitation functions for various dynamical quantities as well as experimental observables from SIS to RHIC energies within the HSD transport approach which is based on string, quark, diquark ( q , q ̄ , qq , q ̄ q ̄ ) and hadronic degrees of freedom without including any explicit phase transition to a quark–gluon plasma (QGP). It is argued that the failure of this more `conventional' approach in comparison to experimental data should indicate the presence of a different phase which might be either attributed to space–time regions of vanishing scalar quark condensate ( 〈q q ̄ 〉 = 0) or to the presence of a QGP phase with strongly interacting partons. We study the K/π ratio, the low mass dilepton enhancement in the invariant mass regime from 0.2–1.2 GeV as well as charmonium suppression for central Au + Au collisions as a function of the bombarding energy and present predictions for these observables as well as hadron rapidity distributions at RHIC energies. Whereas all observables studied within HSD show smooth increasing/decreasing excitation functions, the experimental K +/π + ratio indicates a maximum at 11 A GeV (or above) which is interpreted as a signature for a chirally restored phase in the course of the reaction.