We investigate hadron production as well as transverse hadron spectra from
proton-proton, proton-nucleus and nucleus-nucleus collisions from 2 A⋅GeV
to 21.3 A⋅TeV within two independent transport approaches (HSD and 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+Au (Pb+Pb) collisions at bombarding energies
above ∼ 5 A⋅GeV, 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 SPS and RHIC energies. However, the
maximum in the K+/π+ ratio at 20 to 30 A⋅GeV 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 pre-hadronic/partonic phase of central Au+Au (Pb+Pb)
collisions.Comment: 20 pages, 15 figures, Phys. Rev. C, in pres