Searching for low mass dark matter via phonon creation in superfluid 4He

TL;DR

This paper explores the potential of superfluid helium-4 as a detector for low-mass dark matter, emphasizing phonon production and decay processes to improve detection strategies.

Contribution

It demonstrates the significance of single phonon production for dark matter detection below 1 MeV and analyzes phonon decay into phonon showers due to anomalous dispersion.

Findings

Single phonons are crucial for detecting sub-MeV dark matter.

Phonon decay leads to showers that can enhance detection.

Detection of phonon showers can improve dark matter mass determination.

Abstract

We consider the scattering of dark matter particles from superfluid liquid $^4$He, which has been proposed as a target for their direct detection. Focusing on dark matter masses below ~1 MeV, we demonstrate from sum-rule arguments the importance of the production of single phonons with energies $\omega \lesssim 1$ meV. We show further that the anomalous dispersion of phonons in liquid $^4$He at low pressures [i.e., $d^2\omega(q)/dq^2>0$, where $q$ and $\omega(q)$ are the phonon momentum and energy] has the important consequence that a single phonon will decay over a relatively short distance into a shower of lower energy phonons centered on the direction of the original phonon. Thus the experimental challenge in this regime is to detect a shower of low energy phonons, not just a single phonon. Additional information from the distribution of phonons in such a shower could enhance the determination of the dark matter mass.