Bulk Fermions in Multi-Brane Worlds

TL;DR

This paper investigates bulk fermion fields in multi-brane models with localized gravity, showing how they can naturally generate small neutrino masses and identifying constraints on model parameters due to neutrino phenomenology.

Contribution

It demonstrates the localization of fermion zero modes as right-handed sterile neutrinos in multi-brane models and explores the resulting phenomenological constraints.

Findings

Bulk fermion zero modes can be localized similarly to gravitons.

Ultralight fermion KK modes can exist alongside ultralight gravitons.

Neutrino properties impose severe constraints on model parameters.

Abstract

We study bulk fermion fields in various multi-brane models with localized gravity. The chiral zero mode that these models support can be identified as a right-handed sterile neutrino. In this case small neutrino Dirac masses can naturally appear due to a localization of the bulk fermion zero mode wavefunction, in an analogous way to graviton, without invoking the see-saw mechanism. The conditions and the options for localization are discussed in detail. It is shown that, considering a well motivated five dimensional mass term, the localization behaviour of this mode can resemble the graviton's at least in a region of the parameter space. As a result, the ''+-+'', ''++'' models can support, in addition to the ultralight graviton KK state, an ultralight localized and strongly coupled bulk fermion KK mode. We find that there are severe constraints on the parameter space of ''+-+'' and ''++'' models if the neutrino properties resulting from this light fermion state are to be reasonable. Furthermore, in the case that also the Bigravity scenario is realized the above special KK mode can induce too large mixing between the neutrino and the KK tower sterile modes restricting even more the allowed parameter space.