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 axionnucleon coupling is strong enough, then axions would be emitted from the SN as a burst and, reaching Earth, may excite the oxygen nuclei in water Cherenkov detectors (O-16+a -> O-16*). 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 deexcitation 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 O-16. These results are used to revisit the detectability of axions from SN 1987A in Kamiokande-II.