Neutrino Masses from Large Extra Dimensions

TL;DR

This work proposes that when the Standard Model lives on a 3-brane in a spacetime with large extra dimensions and a TeV-scale fundamental gravity, neutrino masses arise from higher-dimensional physics rather than a traditional see-saw. It presents two robust mechanisms: Dirac masses from bulk right-handed neutrinos that couple weakly to brane states due to volume suppression, and Majorana masses generated by lepton-number breaking on distant branes communicated through bulk fields. The authors derive the parametric forms of the resulting neutrino masses, show how they can lie in the observed $m_\nu$ range for reasonable $M_*$ and dimensionality, and discuss phenomenological constraints, cosmology, and potential signals such as invisible Higgs decays and enhanced neutrino magnetic moments. The work argues that extra-dimensional scenarios can naturally accommodate the atmospheric and solar neutrino data while opening new avenues for collider and sub-mm gravity tests.

Abstract

Recently it was proposed that the standard model (SM) degrees of freedom reside on a $(3+1)$-dimensional wall or ``3-brane'' embedded in a higher-dimensional spacetime. Furthermore, in this picture it is possible for the fundamental Planck mass $\mst$ to be as small as the weak scale $\mst\simeq O(\tev)$ and the observed weakness of gravity at long distances is due the existence of new sub-millimeter spatial dimensions. We show that in this picture it is natural to expect neutrino masses to occur in the $10^{-1} - 10^{-4}\ev$ range, despite the lack of any fundamental scale higher than $\mst$. Such suppressed neutrino masses are not the result of a see-saw, but have intrinsically higher-dimensional explanations. We explore two possibilities. The first mechanism identifies any massless bulk fermions as right-handed neutrinos. These give naturally small Dirac masses for the same reason that gravity is weak at long distances in this framework. The second mechanism takes advantage of the large {\it infrared} desert: the space in the extra dimensions. Here, small Majorana neutrino masses are generated by breaking lepton number on distant branes.