The Lepton Sector of a Fourth Generation

TL;DR

This paper explores how a heavy fourth-generation lepton sector, especially neutrinos, can be naturally realized in warped extra dimension models, resulting in distinct Majorana and Dirac neutrino characteristics.

Contribution

It demonstrates a model where the fourth-generation neutrino is predominantly Dirac, while light neutrinos are Majorana, consistent with experimental constraints.

Findings

Heavy fourth-generation leptons can be achieved within viable parameter space.

Light neutrinos remain mostly Majorana, compatible with observations.

Lepton flavor violation constraints influence model phenomenology.

Abstract

In extensions of the standard model with a heavy fourth generation one important question is what makes the fourth-generation lepton sector, particularly the neutrinos, so different from the lighter three generations. We study this question in the context of models of electroweak symmetry breaking in warped extra dimensions, where the flavor hierarchy is generated by the localization of the zero-mode fermions in the extra dimension. In this setup the Higgs sector is localized near the infrared brane, whereas the Majorana mass term is localized at the ultraviolet brane. As a result, light neutrinos are almost entirely Majorana particles, whereas the fourth generation neutrino is mostly a Dirac fermion. We show that it is possible to obtain heavy fourth-generation leptons in regions of parameter space where the light neutrino masses and mixings are compatible with observation. We study the impact of these bounds, as well as the ones from lepton flavor violation, on the phenomenology of these models.