Sterile Neutrinos with Secret Interactions - Lasting Friendship with Cosmology

TL;DR

This paper reevaluates the viability of sterile neutrinos with secret interactions mediated by a MeV-scale gauge boson, identifying parameter regions compatible with cosmological constraints and implications for dark matter structure.

Contribution

It provides a detailed analysis of sterile neutrino evolution with secret interactions, identifying viable parameter spaces consistent with cosmological data.

Findings

Two viable parameter regions satisfy BBN, CMB, and LSS constraints.

Large A' coupling region may influence dark matter small-scale structure.

Secret interactions can reconcile sterile neutrinos with cosmological observations.

Abstract

Sterile neutrinos with mass ~1 eV and order 10% mixing with active neutrinos have been proposed as a solution to anomalies in neutrino oscillation data, but are tightly constrained by cosmological limits. It was recently shown that these constraints are avoided if sterile neutrinos couple to a new MeV-scale gauge boson A'. However, even this scenario is restricted by structure formation constraints when A'-mediated collisional processes lead to efficient active-to-sterile neutrino conversion after neutrinos have decoupled. In view of this, we reevaluate in this paper the viability of sterile neutrinos with such "secret" interactions. We carefully dissect their evolution in the early Universe, including the various production channels and the expected modifications to large scale structure formation. We argue that there are two regions in parameter space - one at very small A' coupling, one at relatively large A' coupling - where all constraints from big bang nucleosynthesis (BBN), cosmic microwave background (CMB), and large scale structure (LSS) data are satisfied. Interestingly, the large A' coupling region is precisely the region that was previously shown to have potentially important consequences for the small scale structure of dark matter halos if the A' boson couples also to the dark matter in the Universe.