Impact of chirality imbalance and nonlocal interactions on the QCD biased axionic domainwall interpretation of NANOGrav 15 year data

TL;DR

This paper explores how chirality imbalance and nonlocal QCD interactions influence axion-like particle domain walls and their potential to explain the NANOGrav 15-year gravitational wave data, highlighting conditions under which this interpretation is viable.

Contribution

It introduces a nonlocal NJL model analysis of QCD bias effects on axionic domain walls and their gravitational wave signatures, extending previous local models and data interpretations.

Findings

Large chiral chemical potential can make the axionic domain-wall interpretation compatible with NANOGrav data.

The peak of QCD topological susceptibility at the critical temperature shows a pronounced width in the nonlocal model.

QCD bias near the critical temperature can produce gravitational waves consistent with observations.

Abstract

We investigate the influence of the chirality imbalance with local CP-breaking in hot QCD on the generation of a stochastic gravitational wave background (SGWB) sourced by the axion-like particle (ALP) domain-wall annihilation, induced by the QCD bias. Such a bias is quantified by the QCD topological susceptibility $\chi_t$, and its dependence on the $\theta$ angle and the chiral chemical potential $\mu_5$ is investigated at temperatures near the QCD scale within a nonlocal Nambu-Jona-Lasinio (NJL) model. We find that, besides the small-$\theta$ range, the axionic domain-wall interpretation of NANOGrav 15-year data on the nHz gravitational waves is still possible for a certain large-$\theta$ range if $\mu_5$ is large enough. We confirm that the peak of $|\chi_t|$ at the critical temperature $T_c$ for the CP restoration at $\theta=\pi$ exhibits a pronounced width compared to the local NJL result. Thus, for $\theta$ at and around $\pi$, the QCD bias near $T_c$ can also produce a GW signal strength compatible with the NANOGrav 15-year data.