Study of freeze-out dynamics of strange hadrons

We study the chemical freeze-out dynamics of strange particles ($K,\, \Lambda,\, \Sigma$) from a homogeneous and isotropically expanding hadronic system of $\pi, K, \rho, N, \Lambda$ and $\Sigma$ with zero net baryon density. We use the momentum integrated Boltzmann equation and study their evolution over the bulk hadronic matter, a condition being similar to the one created at top RHIC and LHC energies. The cross-sections, which are input to the equations, are taken either from phenomenological models or parameterized by comparing against experimental data. From this microscopic calculation we find that these strange particles freeze-out near transition temperature $\approx T_c$ due to large relaxation time. The continuous cease of the inelastic processes due to gradual fall in the temperature and decrease in the number density, thus lead to early freeze out of strange hadrons $K, \Lambda$ and $\Sigma$ which happens sequentially near $T_c$. However, freeze-out of these strange species near Tc appears as a sudden and simultaneous process, which is mostly predicted by thermal model while explaining the yield of identified particles at RHIC and LHC energies.

Sushant K. Singh, Purabi Ghosh, Jajati K. Nayak · 2020