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

TLDR

Final measurements from SPS and RHIC, together with an updated hadron mass spectrum, motivate this study. The authors present a statistical‑model analysis of hadron production in central heavy‑nucleus collisions. They extend previous models by incorporating high‑mass resonances and the scalar σ meson, yielding a better fit to the data. The refined model reproduces the K⁺/π⁺ “horn” and demonstrates its connection to the QCD phase transition.

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 20–160AGeV) and for the RHIC energies (sNN=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>2GeV), and the up-to-now neglected scalar σ meson leads to an improved description of the data. In particular, the hitherto poorly reproduced energy dependence of the K+/π+ 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.