Searching for low mass dark matter via phonon creation in superfluid
We consider the scattering of dark matter particles from superfluid liquid 4He, 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 ω ≲ 1 meV. We show further that the anomalous dispersion of phonons in liquid 4He at low pressures [i.e., $d^2ω(q)/dq^2 > 0$, where $$\textit{q}$$ and $$\textit{ω(q)}$$ are the phonon momentum and energy] has the important consequence that a single phonon will decay over a relatively short distance into a shower ofmore »