Spontaneous Spin Polarization in Dense Quark Matter Reexamined

Abstract

The phase structure of strongly interacting matter at high baryon density remains an open problem in QCD, with important implications for the internal structure and magnetic properties of neutron stars. We reexamine the emergence of a quark spin-polarized (QSP) phase in cold, dense quark matter within the two-flavor Nambu-Jona-Lasinio model with tensor interactions, focusing on its dependence on regularization. We compare the traditional regularization scheme (TRS), where a sharp momentum cutoff is applied to both vacuum and medium contributions, with the medium separation scheme (MSS), where ultraviolet-divergent vacuum terms are isolated while finite medium-dependent phase-space contributions remain cutoff free. The two prescriptions lead to qualitatively different results. In TRS, the QSP condensate eventually collapses at high chemical potential, leading to nonmonotonic thermodynamic behavior and, for some couplings, violations of causality. In MSS, the QSP condensate instead increases monotonically with density, the equation of state remains causal, and the associated spontaneous magnetization grows toward high density. The persistence of spin polarization at high density may provide a microscopic contribution to the strong magnetic fields of compact stars, including magnetars. An exploratory hybrid-star construction further shows that the QSP phase can significantly modify the equation of state, stability trends, and mass-radius relations. These results highlight the importance of a consistent treatment of medium phase space and demonstrate the potential impact of a persistent spin-polarized quark phase on neutron-star physics.