Hadronic observables at relativistic energies: Anything strange with strangeness?

Abstract

We calculate p, ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},$ ${K}^{\ifmmode\pm\else\textpm\fi{}},$ and $\ensuremath{\Lambda}(+{\ensuremath{\Sigma}}^{0})$ rapidity distributions and compare to experimental data from SIS to SPS energies within the Ultrarelativistic-Quantum-Molecular Dynamics and Hadron-String Dynamics transport approaches that are both based on string, quark, diquark $(q,$ $\overline{q},$ $qq,$ $\overline{q}\overline{q}),$ and hadronic degrees of freedom. The two transport models do not include any explicit phase transition to a quark-gluon plasma. It is found that both approaches agree rather well with each other and with the experimental rapidity distributions for protons, \ensuremath{\Lambda}'s, ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},$ and ${K}^{\ifmmode\pm\else\textpm\fi{}}.$ In spite of this apparent agreement both transport models fail to reproduce the maximum in the excitation function for the ratio ${K}^{+}/{\ensuremath{\pi}}^{+}$ found experimentally between 11 and 40 A GeV. A comparison to the various experimental data shows that this ``failure'' is dominantly due to an insufficient description of pion rapidity distributions rather than missing ``strangeness.'' The modest differences in the transport model results\char22{}on the other hand\char22{}can be attributed to different implementations of string formation and fragmentation that are not sufficiently controlled by experimental data for the ``elementary'' reactions in vacuum.