Localized Neutrinos in Two Dark Dimensions

Abstract

We study neutrino masses in a six-dimensional framework with two compact dark dimensions. In the conventional flat-bulk realization, the sterile-neutrino zero mode is strongly volume suppressed, so a fundamental scale near $10~\mathrm{TeV}$ requires a large higher-dimensional Yukawa coupling to reproduce the observed neutrino masses, while the accompanying two-dimensional Kaluza--Klein tower is strongly constrained by oscillation data. We show that localizing the sterile-neutrino wave function near the Standard-Model brane substantially alleviates both effects. Using a Gaussian localizer, realistic Dirac neutrino masses are obtained with perturbative, order-one Yukawa couplings, while the brane-visible sterile excitations form a shell spectrum with $M_s^2=8μ^2 s$, and squared active--sterile mixings that decrease as $1/s$. The localization scale therefore separates the experimentally relevant sterile-neutrino spectrum from the gravitational compactification scale. We derive the resulting three-flavour structure and study its phenomenological consequences for reactor oscillations and tritium beta decay, including comparisons with current flat-LED and KATRIN constraints.