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
$\bar{f} p \to e^+ n$, 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 $\bar{f} p \to e^+ n$ at interesting rates in neutrino
detectors, as long as $m_f \lesssim m_{\pi}$. 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 $\bar{f}
p \to e^+ n$ in neutrino detectors implied by this operator, and find that
Super-K can probe the new physics scale $\Lambda$ for this interaction up to
${\cal O}(100 {TeV})$
