  We study novel scenarios where thermal dark matter (DM) can be efficiently
captured in the Sun and annihilate into boosted dark matter. In models with
semi-annihilating DM, where DM has a non-minimal stabilization symmetry, or in
models with a multi-component DM sector, annihilations of DM can give rise to
stable dark sector particles with moderate Lorentz boosts. We investigate both
of these possibilities, presenting concrete models as proofs of concept. Both
scenarios can yield viable thermal relic DM with masses O(1)-O(100) GeV. Taking
advantage of the energetic proton recoils that arise when the boosted DM
scatters off matter, we propose a detection strategy which uses large volume
terrestrial detectors, such as those designed to detect neutrinos or proton
decays. In particular, we propose a search for proton tracks pointing towards
the Sun. We focus on signals at Cherenkov-radiation-based detectors such as
Super-Kamiokande (SK) and its upgrade Hyper-Kamiokande (HK). We find that with
spin-dependent scattering as the dominant DM-nucleus interaction at low
energies, boosted DM can leave detectable signals at SK or HK, with sensitivity
comparable to DM direct detection experiments while being consistent with
current constraints. Our study provides a new search path for DM sectors with
non-minimal structure.
