Flavourful Axion Phenomenology

TL;DR

This paper explores the phenomenology of flavourful axions, including their couplings, effects on meson decays, mixing with mesons, and implications for lepton-flavour violation, with a focus on a specific Pati-Salam model.

Contribution

It provides a comprehensive analysis of flavourful axion interactions, meson mixing, and decay processes, and introduces a predictive Pati-Salam model with fixed axion couplings.

Findings

Calculated axion-fermion and axion-photon couplings.

Derived branching ratios for meson decays involving axions.

Predicted correlations between flavour observables in a specific model.

Abstract

We present a comprehensive discussion of the phenomenology of flavourful axions, including both standard Peccei-Quinn (PQ) axions, associated with the solution to the strong $CP$ problem, and non-standard axion-like particles (ALPs). We give the flavourful axion-fermion and axion-photon couplings and calculate the branching ratios of heavy meson ($K$, $D$, $B$) decays involving a flavourful axion. We also calculate the mixing between axions and heavy mesons $ K^0 $, $ D^0 $, $ B^0 $ and $ B_s^0 $, which affects the meson oscillation probability and mass difference. Mixing also contributes to meson decays into axions and axion decays into two photons, and may be relevant for ALPs. We discuss charged lepton flavour-violating decays involving final state axions of the form $\ell_1 \to \ell_2 a (\gamma) $, as well as $ \mu \to eee $ and $ \mu-e $ conversion. Finally we describe the phenomenology of a particular "A to Z" Pati-Salam model, in which PQ symmetry arises accidentally due to discrete flavour symmetry. Here all axion couplings are fixed by a fit to flavour data, leading to sharp predictions and correlations between flavour-dependent observables.