Abstract
Axion models with generation-dependent Peccei-Quinn charges can lead to flavor-changing neutral currents, thus motivating QCD axion searches at precision flavor experiments. We rigorously derive limits on the most general effective flavor-violating couplings from current measurements and assess their discovery potential. For two-body decays we use available experimental data to derive limits on $q\to q' a$ decay rates for all flavor transitions. Axion contributions to neutral-meson mixing are calculated in a systematic way using chiral perturbation theory and operator product expansion. We also discuss in detail baryonic decays and three-body meson decays, which can lead to the best search strategies for some of the couplings. For instance, a strong limit on the $\Lambda\to n a$ transition can be derived from the supernova SN 1987A. In the near future, dedicated searches for $q\to q' a$ decays at ongoing experiments could potentially test Peccei-Quinn breaking scales up to $10^{12}$ GeV at NA62 or KOTO, and up to $10^{9}$ GeV at Belle II or BES III.
Highlights
The QCD axion is arguably one of the best-motivated particles beyond the Standard Model (SM)
Some of the bounds are afflicted by large theoretical uncertainties which could in principle change the quoted numerical values by as much as an order of magnitude
The affected bounds are (i) the bounds on FAsd;V from supernova cooling due to Λ → na transition, where the temperature of the protoneutron star (PNS) and the interpretation of the SN 1987A neutrino events are two important sources of potential systematic errors; (ii) the meson-mixing bounds from ΔmD and from kaon mixing on FVds suffering from poorly known theoretical predictions; and (iii) bounds on top-axion couplings relying on additional, model-dependent assumptions on the absence of cancellations with tree-level contributions
Summary
The QCD axion is arguably one of the best-motivated particles beyond the Standard Model (SM). In the KSVZ-type models [36,37] the color anomaly is due to a set of heavy fermions that are vectorlike under the SM but chiral under the PQ symmetry In this case, the axion does not couple to elementary SM fermions at tree level. Though, flavor-violating axion couplings are present already at tree level This is the case, for instance, in generalized DFSZ-type models with generation-dependent PQ charges [40,41,42,43,44], which can allow one to suppress the axion couplings to nucleons [45,46,47]. Details about renormalization of effective axion couplings, experimental recasts of two-body meson decays, and hadronic inputs are deferred to the Appendix
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