I will discuss the possibility of looking for sub-MeV dark matter using superfluid helium-4, which is currently the most promising proposal for such a mass range. In particular, I will employ the recently developed relativistic effective theory for the superfluid phonon to describe the response of the detector to the passing dark matter. Being formulated in a quantum field theory language, this approach is particularly suitable for a particle physics problem like the one at hand, and presents several advantages with respect to the standard techniques. I will apply the formalism to a model of dark matter with both a scalar and vector mediator, and discuss what kind of constraints such an experiment can put on these model.