Evidence for Nonhadronic Degrees of Freedom in the Transverse Mass Spectra of Kaons from Relativistic Nucleus-Nucleus Collisions?

We investigate transverse hadron spectra from relativistic nucleus-nucleus collisions within two independent transport approaches that are based on quark, diquark, string, and hadronic degrees of freedom. Both transport models show their reliability for elementary $pp$ as well as light-ion (C+C, Si+Si) reactions. However, for central Au+Au (Pb+Pb) collisions above ~5A GeV the measured K^± transverse mass spectra have a larger inverse slope parameter than expected from the calculation. Thus, the pressure generated by hadronic interactions in the transport models above ~5A GeV is lower than observed in the experimental data. This finding shows that the additional pressure---as expected from lattice QCD calculations at finite quark chemical potential and temperature---is generated by strong partonic interactions in the early phase of central Au+Au (Pb+Pb) collisions.