Exciting Prospects for Detecting Late-Time Neutrinos from Core-Collapse Supernovae.

The importance of detecting neutrinos from a Milky Way core-collapse supernova is well known. An under-studied phase is proto-neutron star cooling. For SN 1987A, this seemingly began at about 2 s, and is thus probed by only 6 of the 19 events (and only the $\bar{\nu}_e$ flavor) in the Kamiokande-II and IMB detectors. With the higher statistics expected for present and near-future detectors, it should be possible to measure detailed neutrino signals out to very late times. We present the first comprehensive study of neutrino detection during the proto-neutron star cooling phase, considering a variety of outcomes, using all flavors, and employing detailed detector physics. For our nominal model, the event yields (at 10~kpc) after 10~s---the approximate duration of the SN 1987A signal---far exceed the entire SN 1987A yield, with $\simeq$250 $\bar{\nu}_e$ events (to 50~s) in Super-Kamiokande, $\simeq$110 $\nu_e$ events (to 40~s) in DUNE, and $\simeq$10 $\nu_\mu, \nu_\tau, \bar{\nu}_\mu, \bar{\nu}_\tau$ events (to 20~s) in JUNO. These data would allow unprecedented probes of the proto-neutron star, including the onset of neutrino transparency and hence its transition to a neutron star. If a black hole forms, even at very late times, this can be clearly identified. But will the detectors fulfill their potential for this perhaps once-ever opportunity for an all-flavor, high-statistics detection of a core collapse? Maybe. Further work is urgently needed, especially for DUNE to thoroughly investigate and improve its MeV capabilities.