Detection of the 4.4-MeV gamma rays from 16O(ν, ν′)16O(12.97 MeV, 2-) with a water Cherenkov detector in supernova neutrino bursts

We first discuss and determine the isospin mixing of the two 2(-) states (12.53 MeV and 12.97 MeV) of the(16)O nucleus using inelastic electron scattering data. We then evaluate the cross section of 4.4-MeV gamma rays produced in the neutrino neutral-current (NC) reaction O-16(nu, nu('))O-16(12.97 MeV, 2(-)) in a water Cherenkov detector at a low energy, below 100 MeV. The detection of gamma rays for E-gamma > 5 MeV from the NC reaction O-16(nu, nu('))O-16(E-x > 16 MeV, T = 1) with a water Cherenkov detector in supernova neutrino bursts has been proposed and discussed by several authors previously. In this article, we discuss a new NC reaction channel from O-16(12.97 MeV, 2(-)) producing a 4.4-MeV gamma ray, the cross section of which is more robust and even larger at low energy (E-nu < 25 MeV) than the NC cross section from O-16(E-x > 16 MeV, T = 1). We also evaluate the number of such events induced by neutrinos from supernova explosion which can be observed by the Super-Kamiokande, an Earth-based 32-kton water Cherenkov detector.