A Frequentist analysis of three right-handed neutrinos with GAMBIT

Marcin Chrzaszcz,Marco Drewes,Suraj Krishnamurthy,Christoph Weniger,Tomás E. Gonzalo,Julia Harz

doi:10.1140/epjc/s10052-020-8073-9

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.

Highlights

1.1 MotivationThe observation of neutrino flavour oscillations is one of the strongest hints for the existence of particle physics beyond the Standard Model (SM)
In Appendix A we comment on the details of the implementation in GAMBIT, in Appendix B we explicitly give the expressions for the different observables, in Appendix C we provide details on how we interpret our results in view of the criterion of technical naturalness, and in Appendix D we show the different partial likelihoods
We presented here the first frequentist global analysis of the extension of the Standard Model by three heavy right-handed Majorana neutrinos for a large range of their masses, from 60 MeV to 500 GeV, and for normal and inverted hierarchy of the active neutrino masses

Summary

Introduction

The observation of neutrino flavour oscillations is one of the strongest hints for the existence of particle physics beyond the Standard Model (SM). The oscillations imply that neutrinos have small masses, while the minimal SM predicts that they are massless. At the same time neutrinos are the only elementary fermions that are only known to exist with left handed chirality νL. If right handed neutrinos νR exist, one could immediately add a Dirac mass term νL MDνR to the SM Lagrangian in analogy to all other known fermions. The fact that the νR have not been seen yet could be explained because they are “sterile”, i.e., not charged under any known gauge interactions. The same property makes it possible for them to have a Majorana mass term νR

Objectives

Results

Discussion

Conclusion