On the Detectability of Light Dark Matter with Superfluid Helium
  We show that a two-excitation process in superfluid helium, combined with
sensitivity to meV energy depositions, can probe dark matter down to the ~keV
warm dark matter mass limit. This mass reach is three orders of magnitude below
what can be probed with ordinary nuclear recoils in helium at the same energy
resolution. For dark matter lighter than $\sim 100$ keV, the kinematics of the
process requires the two athermal excitations to have nearly equal and opposite
momentum, potentially providing a built-in coincidence mechanism for
controlling backgrounds.
