QCD axions and domain walls in dense matter under compact stellar conditions

TL;DR

This work investigates QCD axions and domain walls in hot, dense quark matter under compact-star conditions using a two-flavor NJL model extended to include the axion via the U(1)_A anomaly. By enforcing electric charge neutrality and beta equilibrium, it quantifies how temperature and chemical potential modify the QCD topological cumulants, the axion potential, and domain-wall properties through the chiral transition. The study finds that the axion mass is strongly suppressed near chiral restoration, while the self-coupling exhibits a dramatic enhancement near the phase boundary (up to about seven times its vacuum value around T ≈ 70 MeV and μ ≈ 346 MeV), and the domain-wall tension diminishes with increasing T and μ. These results imply that in-medium axion interactions could significantly affect the structure and evolution of compact stars, offering potential observational signatures and guiding future theoretical refinements and lattice comparisons in dense QCD matter.

Abstract

In compact stellar environments, the stability of dense QCD matter requires the simultaneous fulfillment of charge neutrality and beta equilibrium. In this work, we study how temperature and finite chemical potential affect QCD topology and axion properties within this medium, analyzing both cases with and without the charge neutrality condition. Our results show that the topological susceptibility and axion properties are highly sensitive to the critical behavior of the chiral phase transition in both cases. In particular, the axion mass is strongly suppressed near the transition, while the axion self-coupling constant develops a pronounced peak whose magnitude depends on the temperature and density of the medium. Remarkably, around the critical point at $T\simeq70$ MeV and $μ\simeq346$ MeV, the self-coupling constant is enhanced by more than a factor of seven compared to its vacuum value, a feature that to the best of our knowledge has not been reported in previous studies. Such a strong amplification at the phase boundary indicates that axion-mediated interactions could play an important role in shaping the structure and stability of compact stars, with potential implications for their evolution and observable astrophysical signatures.

Figures (7)

• Figure 1: Chiral condensate, scaled by its value in the vacuum, as a function of the chemical potential at different temperatures with (solid) and without (dashed) electric charge neutrality.
• Figure 2: Thermodynamic potential as a function of $\theta$ at different chemical potentials. The potential is measured in units of the momentum cutoff $\Lambda$.
• Figure 3: Domain wall tension as a function of chemical potential at various temperatures, comparing results with (solid) and without (dashed) the electric charge neutrality condition.
• Figure 4: Topological susceptibility, scaled by its value in the vacuum, as a function of the temperature at different temperatures with (solid) and without (dashed) the charge neutrality condition.
• Figure 5: Normalized fourth cumulant, scaled by its value in the vacuum, as a function of the chemical potential at different temperatures with (solid) and without (dashed) the charge neutrality condition.