# A frequentist analysis of three right-handed neutrinos with GAMBIT

## TL;DR

This paper conducts the first comprehensive frequentist analysis of a Standard Model extension with three right-handed neutrinos, covering masses from 60 MeV to 500 GeV, integrating diverse experimental constraints.

## Contribution

It provides the most complete assessment to date of the three right-handed neutrino model below the TeV scale using GAMBIT, including parameter constraints and flavor mixing insights.

## Key findings

- Derived combined profile likelihoods for key parameters.
- Identified statistically preferred regions in parameter space.
- Explored flavor mixing patterns for different lightest neutrino masses.

## Abstract

The extension of the Standard Model by right-handed neutrinos can not only explain the active neutrino masses via the seesaw mechanism, it is also able solve a number of long standing problems in cosmology. Especially, masses below the TeV scale are of particular interest as they can lead to a plethora of signatures in experimental searches. We present the first full frequentist analysis of the extension of the Standard Model by three right-handed neutrinos, with masses between 60 MeV and 500 GeV, using the Global and Modular BSM (beyond the Standard Model) Inference Tool GAMBIT. Our analysis is based on the Casas-Ibarra parametrisation and includes a large range of experimental constraints: active neutrino mixing, indirect constraints from, e.g., electroweak precision observables and lepton universality, and numerous direct searches for right-handed neutrinos. To study their overall effect, we derive combined profile likelihood results for the phenomenologically most relevant parameter projections. Furthermore, we discuss the role of (marginally) statistically preferred regions in the parameter space. Finally, we explore the flavour mixing pattern of the three right-handed neutrinos for different values of the lightest neutrino mass. Our results comprise the most comprehensive assessment of the model with three right-handed neutrinos model below the TeV scale so far, and provide a robust ground for exploring the impact of future constraints or detections.