Seminar, Mohammad Aghaie (Osaka U.)

Seminar, Mohammad Aghaie (Osaka U.)

Title: Anomaly-free axion-like particle in Nelson-Barr models Abstract: The strong CP problem is one of the long-standing puzzles of the Standard Model. The Nelson-Barr mechanism provides an attractive solution through spontaneous CP violation while keeping the QCD vacuum angle effectively zero. In this talk, I will discuss a remarkable low-energy consequence of this mechanism: the natural emergence of a light axion-like particle (ALP). I will first briefly introduce the strong CP problem and the Nelson-Barr mechanism. I will then show how an ALP arises naturally in these models without requiring additional ingredients. Unlike the QCD axion, this ALP is anomaly-free, with highly suppressed couplings to photons and gluons. At the same time, it has flavor-violating couplings to Standard Model quarks, with a structure closely related to the CKM matrix. Rare meson decays can therefore provide an interesting probe of the underlying Nelson-Barr mechanism. I will also discuss the cosmological properties of this ALP, considering both freeze-in and misalignment production, and study the constraints from precision flavor measurements, stellar cooling, and cosmological observations. In particular, I will emphasize how rare flavor-changing decays can probe the high-energy physics responsible for solving the strong CP problem. The combination of precision flavor experiments, astrophysical observations, and future cosmological and X-ray surveys may therefore provide a way to test the Nelson-Barr mechanism through its low-energy ALP signature. (arXiv:2608.20773)