Nucleon Decays into Light New Particles in Neutrino Detectors

Proton and neutron decays into light new particles $X$ can drastically change the experimental signatures and benefit from the complementarity of large water-Cherenkov neutrino detectors such as Super/Hyper-Kamiokande and tracking detectors such as JUNO and DUNE. The proton decays $p\to \ell^+ X$ and $p\to \pi^+ X$ with $m_X$ near phase-space closure lead to charged particles below Cherenkov threshold, rendering them practically invisible in Super- and Hyper-Kamiokande but not in JUNO and DUNE, which are therefore uniquely positioned for these baryon-number-violating signatures despite their smaller size. As an additional signature, such nucleon decays in Earth can produce a sizable flux of $X$ particles in underground detectors. We present a simple model in which nucleons decay into sub-GeV sterile neutrinos that subsequently decay through active-sterile neutrino mixing, with a promisingly large number of events in Super-Kamiokande even in the seesaw-motivated parameter space.