$<m_T>$ excitation function: Freeze-out and equation of state dependence

TL;DR

This study uses a combined Boltzmann+hydrodynamics model to analyze how freeze-out conditions and the equation of state influence the mean transverse mass in heavy-ion collisions across a wide energy range, revealing sensitivity of <m_T> to these factors.

Contribution

It introduces a systematic comparison of freeze-out and equation of state effects on <m_T> using a hybrid UrQMD-based approach without parameter tuning.

Findings

<m_T> is sensitive to freeze-out and EoS variations.

Bag model EoS with phase transition aligns with experimental <m_T> step behavior.

Pure transport models overestimate <m_T> at high energies.

Abstract

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 $E_{\rm lab}=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 < m_T > excitation function. The hybrid model with a hadron gas equation of state leads to a overestimation of the < m_T >, especially at higher energies. However, non-equilibrium effects seem also to be substantial as is suggested by the comparison with standard UrQMD results.