Separate chemical freeze-outs of strange and non-strange hadrons and problem of residual chemical non-equilibrium of strangeness in relativistic heavy ion collisions

TL;DR

This paper develops an advanced hadron resonance gas model with separate chemical freeze-outs for strange and non-strange hadrons, incorporating a strangeness non-equilibrium factor, to accurately fit experimental data and explore signals of quark-hadron phase transition.

Contribution

It introduces a model with separate freeze-outs and a strangeness non-equilibrium factor, improving data fit and analyzing phase transition signals in heavy ion collisions.

Findings

Successful fit of hadron multiplicity ratios with low chi-squared.

Strangeness non-equilibrium factor is near unity, except at 7.6 GeV collision energy.

Detected irregularities in thermodynamic parameters indicating possible phase transition.

Abstract

We present an elaborate version of the hadron resonance gas model with the combined treatment of separate chemical freeze-outs for strange and non-strange hadrons and with an additional $\gamma_{s}$ factor which accounts for the remaining strange particle non-equilibration. Within suggested approach the parameters of two chemical freeze-outs are connected by the conservation laws of entropy, baryonic charge, third isospin projection and strangeness. The developed model enables us to perform a high-quality fit of the hadron multiplicity ratios measured at AGS, SPS and RHIC with $\chi^2/dof \simeq 0.93$. A special attention is paid to a successful description of the Strangeness Horn. The well-known problem of selective suppression of $\bar \Lambda $ and $\bar \Xi$ hyperons is also discussed. The main result is that for all collision energies the $\gamma_{s}$ factor is about 1 within the error bars, except for the center of mass collision energy 7.6 GeV at which we find about 20\% enhancement of strangeness. Also we confirm an existence of strong jumps in pressure, temperature and effective number of degrees of freedom at the stage of strange particle chemical freeze-out, when the center of mass collision energy changes from 4.3 to 4.9 GeV. We argue that these irregularities may signal about the quark-hadron phase transition.