Axion Dark Matter from Heavy Quarks

We propose simple scenarios where the observed dark matter abundance arises from decays and scatterings of heavy quarks through freeze-in of an axion-like particle with mass in the 10 \, keV - 1 \, MeV range. These models can be tested by future X-ray telescopes, and in some cases will be almost entirely probed by searches for two-body decays K → π + invis. at NA62. As a byproduct, we discuss the cancellation of IR divergencies in flavor-violating scattering processes relevant for thermal axion production, and derive the general contribution to axion-photon couplings from all three light quarks.