Mass signature of supernova ${\nu }_{\mu }$ and ${\nu }_{\tau }$ neutrinos in SuperKamiokande

The ${\nu }_{\mu }$ and ${\nu }_{\tau }$ neutrinos (and their antiparticles) from a Galactic core-collapse supernova can be observed in a water-\v{C}erenkov detector by the neutral-current excitation of ${}^{16}$O. The number of events expected is several times greater than from neutral-current scattering on electrons. The observation of this signal would be a strong test that these neutrinos are produced in core-collapse supernovae, and with the right characteristics. In this paper, this signal is used as the basis for a technique of neutrino mass determination from a future Galactic supernova. The masses of the ${\nu }_{\mu }$ and ${\nu }_{\tau }$ neutrinos can either be measured or limited by their delay relative to the ${\bar{\nu }}_e$ neutrinos. By comparing to the high-statistics ${\bar{\nu }}_e$ data instead of the theoretical expectation, much of the model dependence is canceled. Numerical results are presented for a future supernova at 10 kpc as seen in the SuperKamiokande detector. Under reasonable assumptions, and in the presence of the expected counting statistics, ${\nu }_{\mu }$ and ${\nu }_{\tau }$ masses down to about 50 eV can be simply and robustly determined. The signal used here is more sensitive to small neutrino masses than the signal based on neutrino-electron scattering.