Sub-MeV Dark Matter and the Goldstone Modes of Superfluid Helium

Understand

We show how the relativistic effective field theory for the superfluid phase of helium-4 can replace the standard methods used to compute the production rates of low momentum excitations due to the interaction with an external probe.

• This is done by studying the scattering problem of a light dark matter particle in the superfluid, and comparing to some existing results.
• We show that the rate of emission of two phonons, the Goldstone modes of the effective theory, gets strongly suppressed for sub-MeV dark matter particles due to a fine cancellation between two different tree-level diagrams in the limit of small exchanged momenta.
• This phenomenon is found to be a consequence of the particular choice of the potential felt by the dark matter particle in helium.