Excluding MeV-scale QCD axions by KL -> pi0 pi0 a at KTeV

概要

An interesting proposal suggests that a QCD axion (a) coupling to the up quark, down quark, and electron remains viable for an axion mass near 10~MeV. In this paper, this possibility is reexamined by deriving new bounds from kaon decays. In particular, we perform a detailed analysis of the KL -> pi0 pi0 e+ e- measurement reported by the KTeV experiment, and reinterpret K+ decay measurements at the E949 and NA62 experiments to constrain both the diphoton decay and effectively invisible decay modes of the axion.

We find that, combined with the previously known bounds, the viable window for the MeV-scale QCD axion is excluded, primarily due to the KTeV bound. Uncertainties associated with the chiral Lagrangian are further examined, and the scenario remains excluded even after accounting for these uncertainties, except for a tiny region of parameter space where higher-order corrections must finely cancel the leading-order contribution, suppressing the branching ratio of KL->pi0

pi0 a by three orders of magnitude.

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