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 $q\to q^{\prime }a$ 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 $\mathrm{<ruby><rb>\Lambda </rb><rt>らむだ</rt></ruby>}\to na$ transition can be derived from the supernova SN 1987A. In the near future, dedicated searches for $q\to q^{\prime }a$ decays at ongoing experiments could potentially test Peccei-Quinn breaking scales up to ${10}^{12}\mathrm{GeV}$ at NA62 or KOTO and up to ${10}^9\mathrm{GeV}$ at Belle II or BES III.

• Received 20 February 2020
• Accepted 24 June 2020

DOI:https://doi.org/10.1103/PhysRevD.102.015023

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society