On the nature of the fourth generation neutrino and its implications

TL;DR

This paper explores the properties of a hypothetical fourth-generation neutrino within an extended Standard Model, analyzing how its mass and mixing parameters influence light neutrino masses and cosmological constraints.

Contribution

It provides a detailed analysis of the mass mechanisms and constraints for fourth-generation neutrinos, highlighting the implications of Majorana masses and lepton number violation.

Findings

Fourth-generation neutrinos require a light right-handed neutrino.

Majorana masses induce significant two-loop contributions to light neutrino masses.

Constraints on mass parameters and mixings are derived from cosmological bounds.

Abstract

We consider the neutrino sector of a Standard Model with four generations. While the three light neutrinos can obtain their masses from a variety of mechanisms with or without new neutral fermions, fourth-generation neutrinos need at least one new relatively light right-handed neutrino. If lepton number is not conserved this neutrino must have a Majorana mass term whose size depends on the underlying mechanism for lepton number violation. Majorana masses for the fourth generation neutrinos induce relative large two-loop contributions to the light neutrino masses which could be even larger than the cosmological bounds. This sets strong limits on the mass parameters and mixings of the fourth generation neutrinos.