Hyperon longitudinal polarization and vector meson spin alignment in a thermal model for heavy-ion collisions

Abstract

The concept of a common local spin equilibrium for both spin-1/2 and spin-1 particles is incorporated into a thermal model of particle production in heavy-ion collisions at the top RHIC energies. We show that an effective spin polarization tensor leading to a correct description of the longitudinal spin polarization of $\Lambda$ hyperons simultaneously yields a positive alignment of vector mesons ($\phi$ and $K^{*0}$) that grows monotonically with transverse momentum and centrality. Similar trends can be seen in the data, suggesting a possible common mechanism for longitudinal spin polarization and alignment. However, model calculations are insufficient to explain the data in a fully quantitative way. The correlation found between the magnitude of the $\Lambda$ longitudinal polarization and vector meson alignment suggests further more elaborate investigations of this issue.