Probing Axion–Nucleon Interactions with Water Cherenkov Experiments

We discuss the prospects of using large water Cherenkov experiments, such as Hyper-Kamiokande, to probe two fundamental interactions of the QCD axion: the couplings to nucleons. Starting from the general axion-nucleon interaction in a chiral effective field theory framework, we consider axion emission from nearby core-collapse supernovae and their subsequent absorption in water through processes such as $a + N \to N + \gamma$ and $a + N \to N + \pi^0$. The resulting Cherenkov light spectra can generally be separated from the standard neutrino signal. We outline the theoretical setup, the main observables, and the parametric dependence of the expected event rates on the axion couplings and on the supernova distance. The material presented here is based on Ref. [1].