Thermal hadron production in relativistic nuclear collisions: the hadron mass spectrum, the horn, and the QCD phase transition

TL;DR

This paper uses an updated statistical model with an extended hadron mass spectrum to analyze hadron production in heavy-ion collisions, successfully explaining the energy dependence of particle ratios and linking it to the QCD phase transition.

Contribution

It introduces an improved statistical model incorporating high-mass resonances and the scalar sigma meson, enhancing the description of experimental data and connecting the horn to the QCD phase transition.

Findings

Accurately reproduces the energy dependence of the $K^+/\pi^+$ ratio

Links the horn phenomenon to the QCD phase transition

Highlights the importance of the hadron mass spectrum in statistical models

Abstract

We present, using the statistical model, a new analysis of hadron production in central collisions of heavy nuclei. This study is motivated by the availability of final measurements both for the SPS (beam energies 20-160 AGeV) and for the RHIC energies ($\sqrt{s_{NN}}$=130 and 200 GeV) and by updates in the hadron mass spectrum, which is a crucial input for statistical models. Extending previous studies by inclusion of very high-mass resonances (m> 2 GeV), and the up-to-now neglected scalar $\sigma$ meson leads to an improved description of the data. In particular, the hitherto poorly reproduced energy dependence of the $K^+/\pi^+$ ratio at SPS energies ("the horn") is now well described through the connection to the hadronic mass spectrum and, implicitly, Hagedorn's limiting temperature. We thereby demonstrate the intimate connection between the horn and the QCD phase transition.