Centrality Dependence of Chemical Freeze-Out Parameters and Strangeness Equilibration in RHIC and LHC Energies

Abstract

We have estimated centrality variation of chemical freeze-out parameters from yield data at midrapidity of ${\pi }^{\pm }$ , $K^{\pm }$ and $p$ , $\overline{p}$ for collision energies of RHIC (Relativistic Heavy Ion Collider), Beam Energy Scan (RHIC-BES) program, and LHC (Large Hadron Collider). We have considered a simple hadron resonance gas model and employed a formalism involving conserved charges ( $B,Q,S$ ) of QCD for parameterization. Along with temperature and three chemical potentials ( $T,{\mu }_B,{\mu }_Q,{\mu }_S$ ), a strangeness undersaturation factor ( ${\gamma }_S$ ) has been used to incorporate the partial equilibration in the strange sector. Our obtained freeze-out temperature does not vary much with centrality, whereas chemical potentials and ${\gamma }_S$ seem to have a significant dependence. The strange hadrons are found to deviate from a complete chemical equilibrium at freeze-out at the peripheral collisions. This deviation appears to be more prominent as the collision energy decreases at lower RHIC-BES energies. We have also shown that this departure from equilibrium reduces towards central collisions, and strange particle equilibration may happen after a threshold number of participants in $A$ - $A$ collision.