Detectability of thermal neutrinos from binary-neutron-star mergers and implication to neutrino physics

We propose a long-term strategy for detecting thermal neutrinos from the remnant of binary-neutron-star mergers with a future M-ton water-Cherenkov detector such as Hyper-Kamiokande. Monitoring >~2500 mergers within <~200 Mpc, we may be able to detect a single neutrino with a human-time-scale operation of ~80 Mt years for the merger rate of 1 Mpc^{-3} Myr^{-1}, which is slightly lower than the median value derived by the LIGO-Virgo collaboration with GW170817. Although the number of neutrino events is minimal, contamination from other sources of neutrinos can be reduced efficiently to ~0.03 by analyzing only ~1 s after each merger identified with gravitational-wave detectors if Gadolinium is dissolved in the water. The contamination may be reduced further to ~0.01 if we allow the increase of waiting time by a factor of ~1.7. The detection of even a single neutrino can pin down the energy scale of thermal neutrino emission from binary-neutron-star mergers and could strongly support or disfavor formation of remnant massive neutron stars. Because the dispersion relation of gravitational waves are now securely constrained to that of massless particles with a corresponding limit on the graviton mass of <~10^{-22} eV/c^2 by binary-black-hole mergers, the time delay of a neutrino from gravitational waves can be used to put an upper limit of <~O(10) meV/c^2 on the absolute neutrino mass in the lightest eigenstate. Large neutrino detectors will enhance the detectability, and in particular, 5 Mt Deep-TITAND and 10 Mt MICA planned in the future will allow us to detect thermal neutrinos every ~16 and 8 years, respectively, increasing the significance.