Properties of QCD axion in two-flavor color superconductive matter with massive quarks

TL;DR

This paper studies the properties of the QCD axion in two-flavor color superconducting quark matter, revealing how color superconductivity influences axion mass, potential, and domain wall characteristics.

Contribution

It provides analytical dispersion relations and explores the impact of color superconductivity on axion properties within the NJL model at finite density.

Findings

Color superconductivity enhances axion mass and self-coupling.

Axion potential maintains a $ heta$ period of $oldsymbol{ extpi}$ in the superconducting phase.

Surface tension of axion domain walls is affected by color superconductivity.

Abstract

We investigate the properties of QCD axion at low temperature and moderate density in the Nambu-Jona-Lasinio model with instanton induced interactions by simultaneously considering the scalar and pseudo-scalar condensates in both quark-antiquark and diquark channels. We derive the analytical dispersion relations of quarks with four-type condensates at nonzero theta angle $\theta=a/f_a$. The axion mass, quartic self-coupling, and the axion potential are calculated in both the chiral symmetry breaking and two-flavor color superconducting phases. Using the commonly adopted model parameters, we find that due to the emergence of color superconductivity, the chiral phase transition not only does not lead to a significant decrease in axion mass and self-coupling, but rather results in an obvious enhancement of them. As a $\theta$ function, the axion potential exhibits an appropriate period of $\pi$, which is quite different from the case without considering the color superconductivity. The surface tension of axion domain wall is also calculated in the presence of color superconductivity.