Neutrino portals to MeV WIMPs with s-channel mediators

Abstract: Large-scale neutrino detectors currently under construction will have the unique ability to probe the annihilation of low-mass thermal-relic dark matter to neutrinos. This represents an essential test of the thermal freezeout paradigm. This raises the question: what viable UV-complete models are there in which dark matter annihilates dominantly to neutrinos? We discuss models that fulfill this criteria, and are invariant under the Standard Model gauge group, for both scalar and fermionic dark matter. Specifically, we construct new models in which annihilation via the $s$-channel exchange of a scalar or pseudoscalar mediator achieves the correct relic density. In these models, dark matter is stabilised by an exact or a softly-broken lepton-number symmetry. The parameter space of such models will be probed, almost entirely, by the combination of JUNO, Hyper-Kamiokande and CMB-S4.