Hybrid Stars in an SU(3) Parity Doublet Model

We apply an extended version of the SU(3) parity model, containing quark degrees of freedom, to study neutron stars. The model successfully reproduces the main thermodynamic features of QCD which allows us to describe the composition of dense matter. Chiral symmetry restoration is realized inside the star and the chiral partners of the baryons appear, their masses becoming degenerate. Furthermore, quark degrees of freedom appear in a transition to a deconfined state. Performing an investigation of the macroscopic properties of neutron stars, we show that observational constraints, like mass and thermal evolution, are satisfied and new predictions can be made.

💡 Research Summary

The paper presents a comprehensive study of neutron‑star matter using an extended SU(3) parity‑doublet model that incorporates explicit quark degrees of freedom. The authors begin by reviewing the parity‑doublet framework, in which each baryon (e.g., nucleon, hyperon) is paired with a chiral partner of opposite parity. In vacuum the two members have markedly different masses, but as the baryon chemical potential rises the scalar condensates (σ, ζ) melt, restoring chiral symmetry and driving the masses toward degeneracy. This mechanism naturally embeds chiral symmetry restoration into the equation of state (EOS) without invoking an abrupt first‑order transition.

To describe deconfinement, the model is coupled to a Polyakov‑loop field Φ. At low densities Φ≈0 suppresses quark contributions, while above a critical chemical potential Φ rapidly approaches unity, allowing quarks to appear as active degrees of freedom. The quark sector interacts with the same scalar and vector fields as the baryons, ensuring a smooth crossover from hadronic to quark matter. Consequently the resulting EOS exhibits a two‑stage stiffening: first, the appearance of parity partners softens the hadronic phase; second, the onset of deconfined quarks re‑hardens the pressure‑density relation.

Using this EOS the Tolman‑Oppenheimer‑Volkoff equations are solved to obtain mass‑radius (M‑R) curves. For central densities exceeding roughly two to three times nuclear saturation density (ρ₀), a quark core forms, producing so‑called hybrid stars. The calculated maximum masses reach ≳2 M⊙ with radii in the 11–13 km range, comfortably satisfying the most stringent observational constraints, including the 2.14 M⊙ pulsar PSR J0740+6620 and NICER radius measurements. The presence of parity partners reduces the overall stiffness but does not prevent the attainment of high masses because the subsequent quark phase compensates with additional pressure support.

Thermal evolution is examined by integrating the energy‑balance and heat‑transport equations with microphysical inputs that reflect the altered composition. The emergence of parity partners modifies the specific heat and the neutrino‑emission rates (e.g., modified Urca, direct Urca thresholds), while the quark core introduces new cooling channels such as quark direct Urca and pair‑breaking processes. Simulations show that the model reproduces the rapid temperature decline observed in the young neutron star Cassiopeia A, as well as the broader cooling trends of older pulsars. The agreement is achieved without fine‑tuning; it follows directly from the density‑dependent appearance of chiral partners and quarks.

Overall, the study demonstrates that (i) chiral symmetry restoration via parity‑doublet dynamics is a robust feature of dense QCD matter, (ii) deconfinement can be modeled as a smooth crossover within the same effective theory, and (iii) the combined effect yields hybrid stars whose macroscopic observables—mass, radius, and thermal history—are consistent with current astrophysical data. The authors also discuss future prospects: gravitational‑wave signatures from binary neutron‑star mergers could carry imprints of the softening‑hardening sequence, and upcoming heavy‑ion facilities (FAIR, NICA) may probe the relevant density regime, providing complementary laboratory tests of the parity‑doublet–quark framework.