# Ultra-light scalar saving the 3+1 neutrino scheme from the cosmological bounds

## TL;DR

This paper proposes a model where ultra-light scalars couple to sterile neutrinos, preventing their early universe production and alleviating cosmological tensions with eV-scale sterile neutrinos, with implications for upcoming neutrino experiments.

## Contribution

It introduces a novel mechanism involving ultra-light scalars to suppress sterile neutrino production, addressing cosmological bounds and guiding future experimental searches.

## Key findings

- Scalar coupling prevents sterile neutrino production in the early universe
- Model relaxes tension between sterile neutrino hints and cosmological data
- Implications discussed for KATRIN and PTOLEMY experiments

## Abstract

The LSND and MiniBooNE results as well as the reactor and Gallium anomalies seem to indicate the presence of a sterile neutrino with a mass of $\sim 1$ eV mixed with active neutrinos. Such sterile neutrino can be produced in the early universe before the neutrino decoupling, leading to a contribution to the effective number of neutrinos ($N_{eff}$) as well as to a contribution to the sum of neutrino masses which are in tension with cosmological observations. We propose a scenario to relax this tension by a Yukawa coupling of the sterile neutrinos to ultra-light scalar particles which contribute to the dark matter in the background. The coupling induces an effective mass for $\nu_s$ which prevents its production in the early universe. We discuss the implications for the upcoming KATRIN experiment and future relic neutrino search experiments such as PTOLEMY. We also briefly comment on certain non-renormalizable forms of interaction between $\nu_s$ and the scalar and their consequences for the $\nu_s$ production in the early universe.