Energy and system size dependence of chemical freeze-out in relativistic nuclear collisions

TLDR

We investigate chemical freeze‑out in p‑p, C‑C, Si‑Si and Pb‑Pb collisions at 158 A GeV and in Pb‑Pb at 20, 30, 40 and 80 A GeV. Using the statistical hadronization model to analyze hadronic multiplicities, we extract source parameters as functions of participant number and beam energy. The temperature, baryon chemical potential and strangeness saturation vary smoothly with energy and system size, and we provide interpolation formulas for predicting freeze‑out parameters at higher energies, although discrepancies in Pb‑Pb 20–40 A GeV particle ratios remain unexplained by the model.

Abstract

We present a detailed study of chemical freeze-out in p-p, C-C, Si-Si and Pb-Pb collisions at beam momenta of 158A GeV as well as Pb-Pb collisions at beam momenta of 20A, 30A, 40A and 80A GeV. By analyzing hadronic multiplicities within the statistical hadronization model, we have studied the parameters of the source as a function of the number of the participating nucleons and the beam energy. We observe a nice smooth behaviour of temperature, baryon chemical potential and strangeness under-saturation parameter as a function of energy and nucleus size. Interpolating formulas are provided which allow to predict the chemical freeze-out parameters in central collisions at centre-of-mass energies > 4.5 GeV and for any colliding ions. Specific discrepancies between data and model emerge in particle ratios in Pb-Pb collisions at SPS between 20A and 40A GeV of beam energy which cannot be accounted for in the considered model schemes.