Searching for Sub-GeV Dark Matter in the Galactic Centre using
  Hyper-Kamiokande
  Indirect detection of dark matter via its annihilation products is a key
technique in the search for dark matter in the form of weakly interacting
massive particles (WIMPs). Strong constraints exist on the annihilation of
WIMPs to highly visible Standard Model final states such as photons or charged
particles. In the case of s-wave annihilation, this typically eliminates
thermal relic cross sections for dark matter of mass below $\cal{O}$(10) GeV.
However, such limits typically neglect the possibility that dark matter may
annihilate to assumed invisible or hard-to-detect final states, such as
neutrinos. This is a difficult paradigm to probe due to the weak neutrino
interaction cross section. Considering dark matter annihilation in the Galactic
halo, we study the prospects for indirect detection using the Hyper-Kamiokande
(HyperK) neutrino experiment, for dark matter of mass below 1 GeV. We undertake
a dedicated simulation of the HyperK detector, which we benchmark against
results from the similar Super-Kamiokande experiment and HyperK physics
projections. We provide projections for the annihilation cross-sections that
can be probed by HyperK for annihilation to muon or neutrino final states, and
discuss uncertainties associated with the dark matter halo profile. For
neutrino final states, we find that HyperK is sensitive to thermal annihilation
cross-sections for dark matter with mass around 20 MeV, assuming an NFW halo
profile. We also discuss the effects of neutron tagging, and prospects for
improving the reach at low mass.
