Missing-energy searches as a probe of QCD axion and anomalous gauge vectors

Recent measurements of rare meson decays with missing-energy signatures from experiments such as NA62 and Belle II provide strong constraints on new light, weakly-coupled states beyond the Standard Model. While these processes are consistent with neutrino final states, they also offer sensitivity to a wide class of new physics scenarios with invisible particles.
We discuss two representative cases: the QCD axion and, more generally, axion-like particles (ALPs), and light spin-1 vector mediators coupled to anomalous currents. For ALPs, we show how public experimental results can be recast within an effective field theory framework to derive constraints on flavour-violating axion-quark couplings. For the vector case, we consider light gauge bosons coupled to anomalous electroweak currents and discuss the consistency and phenomenological implications of such interactions in EFT descriptions.