Light Dark Matter in Superfluid Helium: Detection with Multi-excitation Production

TL;DR

This paper explores the potential of superfluid helium as a detector for ultra-light dark matter, focusing on multi-excitation processes and their theoretical modeling to enhance detection sensitivity down to keV masses.

Contribution

It develops a detailed theoretical framework for superfluid helium excitations and applies it to dark matter detection, including the effects of multi-excitation production and external electric fields.

Findings

Detection sensitivity extends to keV dark matter masses with multi-excitation processes.

Theoretical calculations match experimental and simulated superfluid helium responses.

External electric fields can enhance the detection rate of hidden photon interactions.

Abstract

We examine in depth a recent proposal to utilize superfluid helium for direct detection of sub-MeV mass dark matter. For sub-keV recoil energies, nuclear scattering events in liquid helium primarily deposit energy into long-lived phonon and roton quasiparticle excitations. If the energy thresholds of the detector can be reduced to the meV scale, then dark matter as light as ~MeV can be reached with ordinary nuclear recoils. If, on the other hand, two or more quasiparticle excitations are directly produced in the dark matter interaction, the kinematics of the scattering allows sensitivity to dark matter as light as ~keV at the same energy resolution. We present in detail the theoretical framework for describing excitations in superfluid helium, using it to calculate the rate for the leading dark matter scattering interaction, where an off-shell phonon splits into two or more higher-momentum excitations. We validate our analytic results against the measured and simulated dynamic response of superfluid helium. Finally, we apply this formalism to the case of a kinetically mixed hidden photon in the superfluid, both with and without an external electric field to catalyze the processes.