# Viable secret neutrino interactions with ultralight dark matter

## TL;DR

This paper explores a model where ultralight dark matter interacts with sterile neutrinos to suppress their early universe production, potentially reconciling laboratory anomalies with cosmological constraints.

## Contribution

It introduces a new mechanism coupling sterile neutrinos to ultralight dark matter, analyzing allowed parameter regions consistent with cosmological bounds.

## Key findings

- Parameter regions compatible with cosmological constraints identified.
- Suppression of sterile neutrino production in the early universe demonstrated.
- Implications for laboratory neutrino oscillation experiments discussed.

## Abstract

Several anomalies in neutrino oscillation experiments point to the existence of a $\sim 1\,$eV sterile neutrino $\nu_s$ mixing with $\nu_e$ at the level of $U_{e4}\cong 0.1$, but such a neutrino is strongly disfavored by constraints on additional light degrees of freedom ($\delta N_{\rm eff}$) and total neutrino mass ($\sum_\nu m_\nu$) from cosmology. "Secret neutrino interactions" that have been invoked to suppress the cosmological production of $\nu_s$ typically falter, but recently it was pointed out that $\nu_s$ could get a large mass in the early universe by coupling to ultralight dark matter $\phi$, which can robustly suppress its production. The model has essentially two free parameters: $m_\phi$, and $m_{s,0}$, the mass of the sterile neutrino at early times, enhanced by its coupling to $\phi$. I determine the parameter regions allowed by limits on $\delta N_{\rm eff}$ and $\sum_\nu m_\nu$ 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.