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 K0, D0, B0 and Bs0, 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 ℓ1 → ℓ2a(γ), as well as μ → eee and μ − 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.
Highlights
It has been realised that the PQ axion need not emerge from an exact global U(1) symmetry, but could result from some discrete symmetry or continuous gauge symmetry leading to an accidental global U(1) symmetry
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)
In this paper we focus on the phenomenology of flavourful axions, including both standard PQ axions, associated with the solution to the strong CP problem, and nonstandard axion-like particles (ALPs)
Summary
2.1 Lagrangian Relevant to a discussion on axion-fermion interactions is the Lagrangian. XfL, xfR are the fermion PQ charges in the left-right (LR) basis, written here as (diagonal) matrices. As xfL, xfR are real, V f and Af (as well as chiral coupling matrices XL,R) are Hermitian. In this formulation, the implications of flavour structure are clear. If xfL = xfR, the U(1)PQ transformation is not chiral (NDW = 0), the Goldstone field a doesn’t couple to the QCD anomaly, the strong CP problem is not solved, and a is interpreted as an ALP.. As long as xfL,fR∝/ I3, we still get flavour-violating (vector and axial) interactions due to weak mixing encoded in ULf,Rf
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