Cross section for supernova axion observation in neutrino water Cherenkov detectors

Axions coupled to nucleons might be copiously emitted from core-collapse supernovae (SNe). If the axion-nucleon coupling is strong enough, axions would be emitted from the SN as a burst and, reaching Earth, may excite the oxygen nuclei in water Cherenkov detectors (${}^{16}{\rm O} + a \to {}^{16}{\rm O}^{*}$). This process will be followed by decay(s) of the excited state resulting in an emission of photons and thus providing a possibility for a direct detection of axions from a Galactic SN in large underground neutrino Cherenkov detectors. Motivated by this possibility, we present an updated calculation of axion-oxygen cross section obtained by using self-consistent continuum Random Phase Approximation. We calculate the branching ratio of the oxygen nucleus de-excitation into gamma-rays, neutrons, protons and $\alpha$-particles and also consider photon emission from secondary nuclei to compute a total $\gamma$ spectrum created when axions excite ${}^{16}{\rm O}$. These results are used to revisit the detectability of axions from SN 1987A in Kamiokande-II.