Probing Sub-GeV Dark Matter with Superfluid Helium and HeRALD

Superfluid helium is a promising target for light dark matter direct detection, with good kinematic matching to low mass dark matter, intrinsic radiopurity, and several promising channels for reading out recoils. We propose HeRALD, the Helium Roton Apparatus for Light Dark matter, an experiment which will search for sub-GeV DM by monitoring superfluid helium for kinetic (phonon and roton) and atomic (singlet and triplet) excitations. TES bolometers immersed in the superfluid will detect singlet photons and triplet excimer deexcitations, while separate TES bolometers suspended above the superfluid surface will detect rotons and phonons when they ‘quantum evaporate’ individual helium atoms from the superfluid, into vacuum, and onto the detector. The binding energy of absorbing these helium atoms to the bolometer surface effectively amplifies quantum evaporation signals, allowing us to potentially reach recoil thresholds below 10 eV. Sensitivity projections indicate that a kg-scale ‘shovel ready’ detector could already explore new parameter space, with the technology ultimately allowing for searches for nuclear recoiling DM as light as 1 MeV. I will discuss the technology and instrumentation behind the HeRALD project, as well as our progress towards realizing a prototype detector.