BBN Constraint on Heavy Neutrino Production and Decay

TL;DR

This paper analyzes how big-bang nucleosynthesis constrains heavy neutrinos, linking their production and decay to their mass and mixing parameters, thereby excluding certain parameter regions.

Contribution

It provides a comprehensive BBN constraint analysis on heavy neutrinos using a minimal model with two parameters, incorporating various early universe effects.

Findings

Identifies BBN excluded regions in heavy neutrino parameter space.

Shows the complementarity of BBN constraints with terrestrial searches.

Provides a predictive framework based on minimal parameters.

Abstract

We explore the big-bang nucleosynthesis (BBN) constraint on heavy neutrino that is a mixture of gauge singlet fermion and active neutrinos in the Standard Model. We work in the minimal model with only two parameters, the heavy neutrino mass $m_4$ and the mixing parameter $|U_{a4}|^2$, where $a=e$, $\mu$, or $\tau$ stands for the active neutrino flavor. We show that both the early universe production mechanism and decay products of the heavy neutrino are determined by $m_4$ and $|U_{a4}|^2$, with little room for further assumptions. This predictivity allows us to present a portrait of the entire BBN excluded parameter space. Our analysis includes various effects including temporary matter domination, energy injections in the form of charged mesons, photons and light neutrinos. The BBN constraint is complementary to terrestrial search for heavy neutrinos (heavy neutral leptons) behind the origin of neutrino masses and portal to the dark sector.