Viable secret neutrino interactions with ultralight dark matter

Several anomalies in neutrino oscillation experiments point to the existence of a $\sim 1\,$eV sterile neutrino $ν_s$ mixing with $ν_e$ at the level of $U_{e4}\cong 0.1$, but such a neutrino is strongly disfavored by constraints on additional light degrees of freedom ($δN_{\rm eff}$) and total neutrino mass ($\sum_νm_ν$) from cosmology. "Secret neutrino interactions" that have been invoked to suppress the cosmological production of $ν_s$ typically falter, but recently it was pointed out that $ν_s$ 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: $m_ϕ$, and $m_{s,0}$, the mass of the sterile neutrino at early times, enhanced by its coupling to $ϕ$. I determine the parameter regions allowed by limits on $δN_{\rm eff}$ and $\sum_νm_ν$ 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.

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