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Global strangeness production in relativistic heavy ion collisions at SPS and RHIC is reviewed. Special emphasis is put on the comparison with the statistical model and the canonical suppression mechanism. It is shown that recent RHIC data on strange particle production as a function of centrality can be explained by a superposition of a fully equilibrated hadron gas and particle emission from single independent nucleon-nucleon collisions in the outer corona.
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Entropy production in the initial compression stage of relativistic heavy-ion collisions from AGS to SPS energies is calculated within a three-fluid hydrodynamical model. The entropy per participating net baryon is found to increase smoothly and does not exhibit a jump or a plateau as in the 1-dimensional one-fluid shock model. Therefore, the excess of pions per participating net baryon in nucleus-nucleus collisions as compared to proton-proton reactions also increases smoothly with beam energy.
Recent extensive data from the beam energy scan of the STAR collaboration at BNL-RHIC provide the basis for a detailed update for the universal behavior of the strangeness suppression factor gamma_s as function of the initial entropy density, as proposed in our recent paper [1]. [1] P. Castorina, S. Plumari and H. Satz, Int. J. Mod. Phys. E26 (2017) 1750081 (arXiv:1709.02706)
Kaon production in pion-nucleon collisions in nuclear matter is studied in the resonance model. To evaluate the in-medium modification of the reaction amplitude as a function of the baryonic density we introduce relativistic, mean-field potentials for the initial, final and intermediate mesonic and baryonic states. These vector and scalar potentials were calculated using the quark-meson coupling (QMC) model. The in-medium kaon production cross sections in pion-nucleon interactions for reaction channels with $Lambda$ and $Sigma$ hyperons in the final state were calculated at the baryonic densities appropriate to relativistic heavy ion collisions. Contrary to earlier work which has not allowed for the change of the cross section in medium, we find that the data for kaon production are consistent with a repulsive $K^+$-nucleus potential.
We explore production mechanism and final state interaction in the $pp to nK^ + Sigma ^ +$ channel based on the inconsistent experimental data published respectively by COSY-11 and COSY-ANKE. The scattering parameter $a> 0$ for $nSigma ^ +$ interaction is favored by large near-threshold cross section within a nonrelativistic parametrization investigation, and a strong $nSigma ^ +$ interaction comparable to $pp$ interaction is also indicated. Based on this analysis we calculate the contribution from resonance $Delta ^* (1920)$ through $pi ^ +$ exchange within resonance model, and the numerical result suggests a rather small near-threshold total cross section, which is consistent with the COSY-ANKE data. With an additional sub-threshold resonance $Delta ^*(1620)$, the model gives a much better description to the rather large near-threshold total cross section published by COSY-11.
The results of the microscopic transport calculations of $bar p$-nucleus interactions within a GiBUU model are presented. The dominating mechanism of hyperon production is the strangeness exchange processes $bar K N to Y pi$ and $bar K N to Xi K$. The calculated rapidity spectra of $Xi$ hyperons are significantly shifted to forward rapidities with respect to the spectra of $S=-1$ hyperons. We argue that this shift should be a sensitive test for the possible exotic mechanisms of $bar p$-nucleus annihilation. The production of the double $Lambda$-hypernuclei by $Xi^-$ interaction with a secondary target is calculated.
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