Neutrinos from failed supernovae at future water and liquid argon detectors

Abstract

We discuss the diffuse flux of electron neutrinos and antineutrinos from cosmological failed supernovae, stars that collapse directly into a black hole, with no explosion. This flux has a hotter energy spectrum compared to regular, neutron-star forming collapses, and therefore it dominates the total diffuse flux from core collapses above 20-45 MeV of neutrino energy. Reflecting the features of the originally emitted neutrinos, the flux of nu_e and anti-nu_e at Earth is larger for larger survival probability of these species, and for stiffer equations of state of nuclear matter. In the energy window 19-29 MeV, the flux from failed supernovae is susbtantial, ranging from 7% to a dominant fraction of the total flux from all core collapses. It can be as large as phi = 0.38 s^{-1} cm^{-2} for anti-nu_e (phi = 0.28 s^{-1} cm^{-2} for nue), normalized to a local rate of core collapses of R_{cc}(0)=10^{-4} yr^{-1} Mpc^{-3}. In 5 years, a 0.45 Mt water Cherenkov detector should see 5-65 events from failed supernovae, while up to 160 events are expected for the same mass with Gadolinium addition. A 0.1 Mt liquid argon experiment should record 1-11 events. Signatures of neutrinos from failed supernovae are the enhancement of the total rates of events from core collapses (up to a factor of 2) and the appearance of high energy tails in the event spectra.