A Monte Carlo of Quasiparticle Dynamics in Superfluid ⁴He for the HeRALD Experiment

We report on building and validating a phonon and roton ('quasiparticle') monte carlo describing the primary signal channel of the HeRALD dark matter experiment. HeRALD is currently in the R&D phase and employs superfluid 4He as a target volume for low-mass (MeV-scale) dark matter. Quasiparticles from low-energy particle recoils propagate to the 4He interfaces (vacuum above and solids otherwise). At the liquid surface these quasiparticles can induce quantum evaporation, and the resulting liberated atoms can be sensed in the vacuum above using low-temperature calorimetry. At the solid interfaces, quasiparticle are reflected or lost, and the physics of this interface is poorly understood. We have built a montecarlo to simulate these quasiparticle processes, and we have begun to compare this model with experimental data to point toward the main avenues of signal loss and the main modes of possible efficiency increase.