Abstract
The theoretical calculation for pseudo-scalars hadronic decays K rightarrow pi a is reviewed. While one-loop penguin contributions are usually considered, tree-level processes have most often been overlooked in literature. Following the Lepage–Brodsky approach the tree-level contribution to the charged and neutral pseudo-scalar decay in ALP is estimated. Assuming generic ALP couplings to SM fermions, the latest NA62/E949 results for the K^+ rightarrow pi ^+ a decay and the present/future KOTO results for the K^0_L rightarrow pi ^0 a decay are used to provide updated bounds on the ALP-fermion Lagrangian sector. Finally, the interplay between the tree-level and one-loop contributions is investigated.
Highlights
Light pseudoscalar particles naturally arise in many extensions of the Standard Model (SM) of particle physics
A detailed analysis of the K → πa decay has been presented, in view of the recent NA62 measurement and the foreseen updates from the KOTO experiment
Assuming flavor and CP conserving Axion-Like Particles (ALPs) couplings with fermions, the dominant contribution to the K → πa decays arises from the penguin diagrams, manly proportional to the ct coupling
Summary
Light pseudoscalar particles naturally arise in many extensions of the Standard Model (SM) of particle physics. In the most constrained scenario, one can assume a unique ALP-fermion coupling, often denoted as caΦ in the literature1 [39,43] These additional assumptions will become useful in simplifying the phenomenological analysis and their implication will be discussed in Sect. 4. A general discussion on tree-level flavor-violating ALPs couplings to fermions can be found in [15,48,49], while for a recent analysis on CP-violating ALP couplings to fermions one is referred, for example, to [50]. A general discussion on tree-level flavor-violating ALPs couplings to fermions can be found in [15,48,49], while for a recent analysis on CP-violating ALP couplings to fermions one is referred, for example, to [50] It might be useful, for simplifying intermediate calculations and explicitly showing the mass dependence of ALPfermion couplings, to write the effective Lagrangian of Eq (3) in the “Yukawa” basis (on the right) instead of the “derivative” one (on the left). The two versions are equivalent up to operators of O(1/ fa2)
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