Sterile neutrino dark matter production in presence of non-standard
  neutrino self-interactions: an EFT approach
  Sterile neutrinos with keV-scale masses are popular candidates for warm dark
matter. In the most straightforward case they are produced via oscillations
with active neutrinos. We introduce effective self-interactions of active
neutrinos and investigate the effect on the parameter space of sterile neutrino
mass and mixing. Our focus is on mixing with electron neutrinos, which is
subject to constraints from several upcoming or running experiments like
TRISTAN, ECHo, BeEST and HUNTER. Depending on the size of the self-interaction,
the parameter space moves closer to, or further away from, the one testable by
those future experiments. In particular, we show that phase 3 of the HUNTER
experiment would test a larger amount of parameter space in the presence of
self-interactions than without them. We also investigate the effect of the
self-interactions on the free-streaming length of the sterile neutrino dark
matter, which is important for structure formation observables.
