Role of Σ^*(1385) on $Λ$ hyperon polarization in relativistic heavy ion collisions

The effect of Σ^*(1385) baryon resonance on the time evolution of the $Λ$ hyperon polarization in hadronic matter is studied using a kinetic approach. This approach explicitly includes the production of the Σ^* resonance from the $Λ-π$ and $Σ(1192)-π$ scatterings as well as its decay into $Λ+π$ or $Σ+π$. The resulting coupled kinetic equations governing the time evolution of $Λ$, $Σ$ and Σ^* numbers and polarizations are solved for Au-Au collisions at √sNN=7.7 GeV and 20-50% centrality, using initial values determined by thermal yields and the thermal vorticity at chemical freeze-out temperature. As the hadronic matter expands and cools, the $Λ$ polarization is found to increase slightly during early times and then decreases very slowly afterwards, while the $Σ$ polarization remains nearly constant and the Σ^* polarization continuously decreases. Including feed-down contributions to the $Λ$ polarization from the decays of partially polarized Σ⁰, Σ^*, and $Ξ(1322)$ hyperons, where the $Ξ$ polarization is obtained by solving coupled kinetic equations for the $Ξ$ and Ξ^*(1532) system, the resulting $Λ$ polarization becomes smaller and decreases over time. In both cases, however, the time variation of the $Λ$ polarization is sufficiently small to support the assumption of an early freeze-out of $Λ$ spin degree of freedom in relativistic heavy ion collisions.