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Description
Spin polarization in nuclear matter has been recognized as a key ingredient in the description of highly vortical systems formed in heavy-ion collisions, motivating detailed studies of the associated phase structure under extreme conditions. Recently, a spin potential, $\mu_{\Sigma}$, has been proposed in the context of lattice quantum chromodynamics (LQCD) as a quantity that measures the tendency of quark spins to align along a preferred direction. Within LQCD, this concept has been explored in setups without gluonic degrees of freedom and is interpreted as a new thermodynamic quantity, allowing its straightforward implementation in effective quark models and facilitating direct comparisons with lattice results.
In this work, we study the effects of spin polarization within the two-flavor entangled Polyakov--Nambu--Jona-Lasinio (EPNJL) model at finite temperature, using the mean-field approximation. We find that the effective quark masses decrease as a function of the spin potential, an effect that is further enhanced by increasing temperature. The resulting phase diagram in the $T \times \mu_{\Sigma}$ plane exhibits a crossover at sufficiently low spin potential and high temperatures, which is separated from first-order phase transition lines at higher spin potentials by a critical endpoint. Our results are in qualitative agreement with recent LQCD findings and predictions from renormalizable models.
