Dirac vs. Majorana neutrino masses from a TeV interval

We investigate the nature (Dirac vs. Majorana) and size of left-handed neutrino masses in a supersymmetric five-dimensional model compactified in the interval [0, π R], where quarks and leptons are localized on the boundaries while the gauge and Higgs sectors propagate in the bulk of the fifth dimension. Supersymmetry is broken by Scherk-Schwarz boundary conditions and electroweak breaking proceeds through radiative corrections. Right-handed neutrinos propagate in the bulk and have a general five-dimensional mass M, which localizes the zero modes towards one of the boundaries, and arbitrary boundary terms. We have found that for generic boundary terms left-handed neutrinos have Majorana masses. However for specific boundary configurations left-handed neutrinos are Dirac fermions as the theory possesses a conserved global U (1) symmetry which prevents violation of lepton number. The size of neutrino masses depends on the localization of the zero-modes of right-handed neutrinos and/or the size of the five-dimensional neutrino Yukawa couplings. Left-handed neutrinos in the sub-eV range require either M R ∼ 10 or Yukawa couplings ∼ 10-3 R, which make the five-dimensional theory perturbative up to its natural cutoff. © 2008 Elsevier B.V. All rights reserved.