Hyperon Longitudinal Polarization and Vector Meson Spin Alignment in a Thermal Model for Heavy-ion Collisions

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 \({\mit \Lambda }\) 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 \({\mit \Lambda }\) longitudinal polarization and vector meson alignment suggests further more elaborate investigations of this issue. Published by the Jagiellonian University 2026 authors