Viable secret neutrino interactions with ultralight dark matter

Abstract

Several anomalies in neutrino oscillation experiments point to the existence of a ∼1 eV sterile neutrino νs mixing with νe at the level of Ue4≅0.1, but such a neutrino is strongly disfavored by constraints on additional light degrees of freedom (δNeff) and total neutrino mass (∑νmν) from cosmology. “Secret neutrino interactions” that have been invoked to suppress the cosmological production of νs typically falter, but recently it was pointed out that νs could get a large mass in the early universe by coupling to ultralight dark matter ϕ, which can robustly suppress its production. The model has essentially two free parameters: mϕ, and ms,0, the mass of the sterile neutrino at early times, enhanced by its coupling to ϕ. I determine the parameter regions allowed by limits on δNeff and ∑νmν from the cosmic microwave background and big bang nucleosynthesis, using a simplified yet accurate treatment of neutrino oscillations in the early universe. This mechanism could have an important impact on laboratory experiments that suggest oscillations with sterile neutrinos.