Dirac Neutrino Masses with Planck Scale Lepton Number Violation

It is shown how pure Dirac neutrino masses can naturally occur at low energies even in the presence of Planck scale lepton number violation. In a 5D setting this is achieved by explicitly breaking the lepton number on the Planck brane while the right-handed neutrino is localized on the TeV brane. A small wave function overlap then naturally leads to a small Dirac Yukawa coupling. In the 4D dual description the right-handed neutrino is identified as a composite CFT bound state and lepton number violation is highly suppressed by CFT operators with large anomalous dimensions.