Strangeness production at SPS energies in a (3+1)-dimensional Boltzmann+hydrodynamics approach

Abstract

Abstract. Results on strange particle production at energies from √ s = 2 − 200 A GeV from a (3+1)dimensional Boltzmann+hydrodynamics hybrid model are presented and compared to results from the Ultra-relativistic Quantum Molecular Dynamics transport approach and existing experimental data. The hybrid model with an intermediate hydrodynamical evolution for the hot and dense stage of the collision allows us to explore the centrality and system size dependence of (multi-strange) hyperon production in A+A collisions. The transition from a system which is not fully thermalized, like in C+C or Si+Si collisions, to supposedly fully thermalized systems as e.g. created in Pb+Pb reactions is investigated. The hyperon yields are enhanced in the hybrid approach due to the local equilibrium assumption and therefore in line with the experimental data. The excitation functions of strange to nonstrange particle yields (e.g. K + /π + ) calculated in the hybrid approach are able to reproduce the famous 'horn' structure at SPS energies with a purely hadronic equation of state in contrast to the pure transport approach, which failed to explain the experimental data.

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“…This implies that contrary to an isothermal transition particles are emitted over a range of temperatures/densities. While this may no be a standard procedure it has been shown in several publications [49,[62][63][64][65][66] that this hybrid model is very well suited to describe particle ratios and yields over a wide range of energies. In addition, recent comparisons of an iso-thermal 3+1 dimensional hypersurface [67] indicate that deviations only occur at large transverse momenta (p ⊥ > 1 GeV), corresponding in our freeze out prescription to very cold and fast cells which are being treated as equilibrated for an extended period of time.…”

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“…Our study is based on a hybrid model which combines viscous fluid dynamical description of the QGP stage with a realistic kinetic simulation of the hadronic stage [40][41][42][43][44][45]. Relativistic hydrodynamics is the most relevant framework to understand the transport properties of the QGP since it directly connects the collective flow developed during the QGP stage with its equation of state.…”