〈mT 〉 excitation function: Freeze-out and equation of state dependence

An integrated Boltzmann+hydrodynamics transport approach is applied to investigate the dependence of the mean transverse mass on the freeze-out and the equation of state over the energy range from Elab = 2 − 160A GeV. This transport approach based on the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) with an intermediate hydrodynamic stage allows for a systematic comparison without adjusting parameters. We find that the multiplicities of pions and protons are rather insensitive to different freeze-out prescriptions and changes in the equation of state, but the yields are slightly reduced in the hybrid model calculation compared to a pure transport calculation while the (anti)kaon multiplicities are increased. The mean transverse mass excitation functions of all three particle species are found to be sensitive to the different freeze-out treatments as well as to the equation of state. We find that the bag model equation of state with a strong first order phase transition is in qualitative agreement with the experimentally observed step-like behaviour in the 〈mT 〉 excitation function. The hybrid model with a hadron gas equation of state leads to a overestimation of the 〈mT 〉, especially at higher energies. However, nonequilibrium effects seem also to be substantial as is suggested by the comparison with standard UrQMD results. PACS numbers: 25.75.-q,25.75.Nq,24.10.Lx,24.10.Nz 〈mT 〉 excitation function 2