Sterile neutrino dark matter within the $\nu$SMEFT

Sterile neutrinos with masses at the $\mathrm{keV}$ scale and mixing to the active neutrinos offer an elegant explanation of the observed dark matter (DM) density. However, the very same mixing inevitably leads to radiative photon emission and the non-observation of such peaked $X$-ray lines rules out this minimal sterile neutrino DM hypothesis. We show that in the context of the Standard Model effective field theory with sterile neutrinos ($\nu$SMEFT), higher dimensional operators can produce sterile neutrino DM in a broad range of parameter space. In particular, $\nu$SMEFT interactions can open the large mixing parameter space due to their destructive interference, through operator mixing or matching, in the $X$-ray emission. We also find that, even in the zero mixing limit, the DM density can always be explained by $\nu$SMEFT operators. The testability of the studied $\nu$SMEFT operators in searches for electric dipole moments, neutrinoless double beta decay, and pion decay measurements is discussed.