Effects of fermionic singlet neutrinos on high- and low-energy observables

TL;DR

This thesis explores how fermionic singlet neutrinos in the inverse seesaw model affect both high- and low-energy observables, revealing new constraints and potential signals for physics beyond the Standard Model.

Contribution

It investigates the inverse seesaw mechanism's impact on lepton flavor universality, rare decay processes, and Higgs boson constraints, especially within supersymmetric extensions.

Findings

Ratios R_K and R_π constrain inverse seesaw parameters.

Enhanced lepton flavor violation in supersymmetric inverse seesaw.

Invisible decay channels weaken constraints on light Higgs bosons.

Abstract

In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings ($\mathcal{O}(1)$) while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios $R_K$ and $R_\pi$ provide new, additional constraints on the inverse seesaw. We have also considered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and $Z^0$ bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.