Dynamical Minimal Flavour Violating Inverse Seesaw

TL;DR

This paper presents a dynamic realization of the Inverse Seesaw mechanism within the Minimal Lepton Flavour Violation framework, introducing a Majoraxion that addresses the Strong CP problem and Dark Matter, with phenomenological implications for neutrino mixing and collider detection.

Contribution

It introduces a novel dynamical approach to the Inverse Seesaw within the MFV context, linking lepton number breaking to a global symmetry and exploring its phenomenology.

Findings

Heavy neutral leptons could be detected at future colliders.

The W mass measurement constrains heavy neutrino masses to 2-3 TeV.

Deviations from unitarity in the PMNS matrix are analyzed.

Abstract

The Inverse Seesaw mechanism is dynamically realised within the Minimal Lepton Flavour Violation context. Lepton number, whose breaking is spontaneously realised, is generalised to a global Abelian factor of the whole flavour symmetry, that also plays the role of the Peccei-Quinn symmetry. The associated Goldstone boson is a Majoraxion that solves the Strong CP problem and represents a Dark Matter candidate. Three distinct scenarios are identified in terms of flavour symmetry and transformation properties of the exotic neutral leptons that enrich the Standard Model spectrum. The associated phenomenology is studied, focusing on the deviations from unitarity of the PMNS mixing matrix. The strongest constraints arise from the determination of the number of active neutrinos through the invisible width of the $Z$, the comparison of the measured $W$ boson mass with its prediction in terms of the Fermi constant from muon decay, and the null searches for the radiative rare muon decay and $\mu\to e$ conversion in nuclei. The heavy neutral leptons may have masses of a few TeV, leaving open the possibility for a direct detection at future colliders. The impact of the recent measurement of the $W$ mass at the CDF II detector has also been considered, which, in one of the scenarios, points to a sharp prediction for the masses of the heavy neutral leptons at about $2-3$ TeV.