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

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.

Comments: 15 pages. This is greatly improved and essentially extended version of arXiv:1312.5149. The title and list of authors are undated