Anomaly-free axion-like particle in Nelson-Barr models

Abstract

We study Nelson--Barr models with a discrete $Z_N$ symmetry that solve the strong CP problem through spontaneous CP violation, and show that they naturally predict a light axion-like particle (ALP) without introducing any additional ingredients. Unlike the QCD axion, this ALP is anomaly-free: its couplings to photons and gluons are highly suppressed, rendering it naturally long-lived. Instead, it couples to quarks through flavor-violating interactions whose structure is dictated by the CKM matrix. These interactions induce rare meson decays, providing a unique probe of the Nelson--Barr mechanism. We study the cosmological production of the ALP through both freeze-in and misalignment mechanisms. We show that the parameter space in which the observed relic abundance is explained by the freeze-in mechanism is subject to stringent constraints from precision flavor experiments and stellar cooling bounds from SN1987A, leaving only a small viable region that will be comprehensively tested by future structure-formation observations such as the Vera Rubin Observatory and next-generation X-ray missions like Athena, GECCO and THESEUS. In contrast, misalignment production remains a robust and viable mechanism for explaining the observed dark matter abundance over a broad region of parameter space. Our results demonstrate that precision flavor measurements, cosmological observations, and X-ray searches provide complementary probes of this anomaly-free ALP and, consequently, of the Nelson--Barr solution to the strong CP problem.