Strangeness dynamics and transverse pressure in relativistic nucleus-nucleus collisions

We investigate hadron production as well as transverse hadron spectra from proton-proton, proton-nucleus, and nucleus-nucleus collisions from 2A0.3em0exGeV to 21.3A0.3em0exTeV within two independent transport approaches, i.e., hadron-string dynamics (HSD) and ultrarelativistic quantum molecular dynamics (UrQMD) that are based on quark, diquark, string, and hadronic degrees of freedom. The comparison to experimental data on transverse mass spectra from $pp$, $pA$, and C+C (or Si+Si) reactions shows the reliability of the transport models for light systems. For central Au+Au0.3em0ex(Pb+Pb) collisions at bombarding energies above ~5A0.3em0exGeV, furthermore, the measured K^± transverse mass spectra have a larger inverse slope parameter than expected from the default calculations. We investigate various scenarios to explore their potential effects on the K^± spectra. In particular the initial state Cronin effect is found to play a substantial role at top Super Proton Synchrotron (SPS) and Relativistic Heavy Ion Collider (RHIC) energies. However, the maximum in the K⁺∕π⁺ ratio at 20--300.3em0exA0.3em0exGeV is missed by $40%$ and the approximately constant slope of the K^± spectra at SPS energies is not reproduced either. Our systematic analysis suggests that the additional pressure---as expected from lattice QCD calculations at finite quark chemical potential μq and temperature T---should be generated by strong interactions in the early prehadronic∕partonic phase of central Au+Au0.3em0ex(Pb+Pb) collisions.