Neutrinos from failed supernovae at future water and liquid argon detectors

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 the flux from 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 ν _e and ν _̄e at Earth is larger when the survival probability of these species is larger, and also when the equations of state of nuclear matter are stiffer. In the 19-29 MeV energy window, the flux from failed supernovae is substantial, ranging from ∼7% to a dominant fraction of the total flux from all core collapses. It can be as large as ēBH=0.38s ^-1cm ^-2 for ν _̄e and as large as eBH=0.28s ^-1cm ^-2 for ν _e, normalized to a local rate of core collapses of R _cc(0)=10 ^-4yr ^-1Mpc ^-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 added. 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.