Neutrino masses and mixings in non-factorizable geometry

TL;DR

This paper investigates bulk fermions in the Randall–Sundrum localized gravity setup, showing that zero modes exist for any 5D mass and that a right-handed zero mode can be localized on the hidden brane if $m>k/2$, enabling small Dirac neutrino masses through brane Yukawa couplings without a see-saw mechanism. To satisfy parity anomaly cancellation, an even number of bulk fermions is required, which naturally yields a hierarchical neutrino mass spectrum and large mixing angles. A minimal two-bulk-fermion model provides a massless state and two light Dirac neutrinos with masses and mixings compatible with solar and atmospheric neutrino oscillation data. The framework makes distinctive predictions for neutrino masses and mixings and links them to the geometry of extra dimensions and the localization of bulk fields.

Abstract

We study bulk fermion fields in the localized gravity model with non-factorizable metric recently proposed by Randall and Sundrum, and Gogberashvili. In addition to a tower of weak-scale Kaluza-Klein states we find a zero mode for any value of the fundamental fermion mass. If the fermion mass is larger than half the curvature of the compact dimension, the zero mode can be localized on the ``hidden'' 3-brane in the Randall-Sundrum model. Identifying this mode with a right-handed neutrino provides a new way for obtaining small Dirac neutrino masses without invoking a see-saw mechanism. Cancellation of the parity anomaly requires introducing an even number of bulk fermions. This naturally leads to a strong hierarchy of neutrino masses and generically large mixing angles.