Probing Sub-GeV Dark Matter with Superfluid Helium

I will describe a dark matter detection technology that will be sensitive to nuclear recoils of sub-GeV dark matter, using superfluid helium as a target. I will briefly review the state of the field of direct dark matter detection, describe motivations to search for sub-GeV dark matter particles and then explain the merits of superfluid helium as a detector material. These include good kinematic matching to low mass dark matter, excellent intrinsic radiopurity, and its unique ability to be cooled down as a liquid to milli-Kelvin temperatures. We propose to read out the recoil signals by calorimetry based on transition edge sensor readout. Calorimeters submerged in the liquid will measure prompt scintillation photons with near-100% efficiency, while the long-lived rotons and phonon excitations will be detected by quantum evaporation of helium atoms from the liquid surface, into vacuum, and then onto a calorimeter array. The binding energy from helium absorption to the calorimeter surface allows for the amplification of these quantum evaporation signals, allowing sub-eV recoil energy thresholds. Taking into account the relevant backgrounds and detector discrimination power based on the light:heat ratio, sensitivity projections show that a small detector (~kg scale) can already explore new parameter space.