A promising technology concept for sub-GeV dark matter detection is described, in which lowtemperature microcalorimeters serve as the sensors and superfluid ~4He serves as the target material. We name this concept "HeRALD," helium roton apparatus for light dark matter. 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, extreme intrinsic radiopurity, high impedance to external vibration noise, and a unique "quantum evaporation" signal channel enabling the detection of phononlike modes via liberation of ~4He atoms into a vacuum. 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 calorimeter 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 technology's recoil energy threshold below the calorimeter energy threshold.We describe signal production and signal sensing probabilities, and estimate the resulting electron recoil discrimination. We simulate radioactive backgrounds from gamma rays and construct an overall background spectrum expectation also including neutrons and solar neutrinos. Finally, we calculate projected sensitivities to dark matter-nucleon elastic scattering, demonstrating that even very small (sub-kg) target masses can probe wide regions of as-yet untested dark matter parameter space.
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