# Hiding an elephant: heavy sterile neutrino with large mixing angle does not contradict cosmology

## TL;DR

This paper proposes a model where keV-scale sterile neutrinos with large mixing angles can exist without conflicting with cosmological constraints, especially when coupled to a hidden sector, enabling new experimental detection possibilities.

## Contribution

It introduces a hidden sector framework that evades standard cosmological bounds on sterile neutrinos, allowing for laboratory detection of heavy sterile neutrinos with large mixing angles.

## Key findings

- Hidden sector coupling suppresses sterile neutrino production in the early Universe.
- Sterile neutrinos can be massless or superheavy in the early Universe within this model.
- Oscillation-based generation of sterile neutrinos is significantly suppressed.

## Abstract

We study a model of a keV-scale sterile neutrino with a relatively large mixing with the Standard Model sector. Usual considerations predict active generation of such particles in the early Universe, which leads to constraints from the total Dark Matter density and absence of X-ray signal from sterile neutrino decay. These bounds together may deem any attempt of creation of the keV scale sterile neutrino in the laboratory unfeasible. We argue that for models with a hidden sector coupled to the sterile neutrino these bounds can be evaded, opening new perspectives for the direct studies at neutrino experiments such as Troitsk $\nu$-mass and KATRIN. We estimate the generation of sterile neutrinos in scenarios with the hidden sector dynamics keeping the sterile neutrinos either massless or superheavy in the early Universe. In both cases the generation by oscillations from active neutrinos in plasma is suppressed.