Viable secret neutrino interactions with ultralight dark matter

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 ($\delta N_{\mathrm{eff}}$) 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 $\varphi$, which can robustly suppress its production. The model has essentially two free parameters: $m_{\varphi }$, and $m_{s,0}$, the mass of the sterile neutrino at early times, enhanced by its coupling to $\varphi$. I determine the parameter regions allowed by limits on $\delta N_{\mathrm{eff}}$ 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.

Comments: 5 pages, 1 figure, submitted to PLB. v2: corrected ${\nu }_e$ damping rate and thermal self-energy, added discussion of spectral distortion and references