Neutrinoless Double Beta Decay from Singlet Neutrinos in Extra Dimensions
We study the model-building conditions under which a sizeable $0\nu\beta\beta$-decay signal to the recently reported level of~0.4 eV is due to Kaluza--Klein singlet neutrinos in theories with large extra dimensions. Our analysis is based on 5-dimensional singlet-neutrino models compactified on an $S^1/Z_2$ orbifold, where the Standard--Model fields are localized on a 3-brane. We show that a successful interpretation of a positive signal within the above minimal 5-dimensional framework would require a non-vanishing shift of the 3-brane from the orbifold fixed points by an amount smaller than the typical scale (100 MeV)$^{-1}$ characterizing the Fermi nuclear momentum. The resulting 5-dimensional models predict a sizeable effective Majorana-neutrino mass that could be several orders of magnitude larger than the light neutrino masses. Most interestingly, the brane-shifted models with only one bulk sterile neutrino also predict novel trigonometric textures leading to mass scenarios with hierarchical active neutrinos and large $\nu_\mu$-$\nu_\tau$ and $\nu_e$-$\nu_\mu$ mixings that can fully explain the current atmospheric and solar neutrino data.