Hidden MeV-scale dark matter in neutrino detectors

The possibility of direct detection of light fermionic dark matter in neutrino detectors is explored from a model-independent standpoint. We consider all operators of dimension six or lower which can contribute to the interaction fp -> e(+)n, where f is a dark Majorana or Dirac fermion. Constraints on these operators are then obtained from the f lifetime and its decays which produce visible gamma rays or electrons. We find one operator which would allow fp -> e(+)n at interesting rates in neutrino detectors, as long as m(f)less than or similar to m(pi). The existing constraints on light dark matter from relic density arguments, supernova cooling rates, and big-bang nucleosynthesis are then reviewed. We calculate the cross section for fp -> e(+)n in neutrino detectors implied by this operator, and find that Super-Kamiokande can probe the new physics scale Lambda for this interaction up to O(100 TeV).