Abstract
We examine the low-energy signatures of axion-like particles (ALPs) in lep- ton flavor violating (LFV) processes. By using a dimension-5 effective Lagrangian, we compute the most general ALP contributions to LFV decays of leptons and mesons. The provided expressions are valid for any choice of ALP mass and couplings. We explore the complementarity of different processes, identifying specific patterns to be experimentally tested. Constraints on LFV couplings are derived from existing data and prospects for forthcoming experiments are also discussed. As a by-product, we revisit the possibility of a simultaneous explanation of the observed discrepancies in the muon and electron g − 2 through ALP interactions.
Highlights
We have explored the signatures of axion-like particles (ALPs) in lepton flavor violating (LFV) observables at low energies
By using the most general dimension-5 effective Lagrangian, which accounts for the ALP couplings to Standard Model (SM) fermions and gauge bosons, we have derived complete expressions for the most relevant LFV decays of leptons and hadrons
These general formulae can be applied for any ALP mass, as well as for any choice of ALP couplings, generalising and complementing previous results available in the literature [47,48,49]
Summary
The most general dimension-5 effective Lagrangian describing ALP interactions with fermions and photons/gluons reads [18]. Where the invariance under the gauge symmetry SU(3)c × U(1)em has been imposed In this equation, f ∈ {u, d, } denotes SM fermions in the mass basis, i, j stand for flavor indices and the dual field strengths are defined as. The effective couplings to photons and gluons are denoted by cγγ and cgg, while vifj and afij stand for the vector and axial-vector ALP couplings to SM fermions, and Λ for the EFT cutoff.. We will denote the effective couplings accounting for the full one-loop contributions as cγγ ≡ ceγffγ and cgg ≡ cegffg. If Λ = 1 TeV an√d cgg (cγγ) = 1, from eq (2.4) we learn that our EFT remains Yukawa unitary up to energies couplings vifj, and afij do s not. Diagrams illustrating the ALP contributions to j → iγ at linear (left panel) and quadratic (right panel) order in the Yukawa couplings
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