A promising technology concept for sub-GeV dark matter detection is
described, in which low-temperature microcalorimeters serve as the sensors and
superfluid 4He serves as the target material. A superfluid helium target has
several advantageous properties, including a light nuclear mass for better
kinematic matching with light dark matter particles, copious production of
scintillation light, extremely good intrinsic radiopurity, a high impedance to
external vibration noise, and a unique mechanism for observing phonon-like
modes via liberation of 4He atoms into a vacuum (`quantum evaporation'). In
this concept, both scintillation photons and triplet excimers are detected
using calorimeters, including calorimeters immersed in the superfluid. Kinetic
excitations of the superfluid medium (rotons and phonons) are detected using
quantum evaporation and subsequent atomic adsorption onto a microcalorimeter
suspended in vacuum above the target helium. The energy of adsorption amplifies
the phonon/roton signal before calorimetric sensing, producing a gain mechanism
that can reduce the techonology's recoil energy threshold below the calorimeter
energy threshold. We describe signal production and signal sensing
probabilities, and estimate electron recoil discrimination. We then simulate
radioactive backgrounds from gamma rays and neutrons. Dark matter - nucleon
elastic scattering cross-section sensitivities are projected, demonstrating
that even very small (sub-kg) target masses can probe wide regions of as-yet
untested dark matter parameter space
