Majorana neutrinos from warped extra dimensions
  Field theory in the presence of extra dimensions has offered interesting solutions to the gauge hierarchy problem, and inspired many attempts to understand the fermion mass generation mechanism. We study the behaviour of a bulk fermion field subject to non-standard boundary conditions under Z2 symmetry in a slice of (5D) AdS. These pseudo-Majorana boundary conditions relate the 5d spinor fields to their 5d charge conjugate and generate Majorana spinors in four-dimensions. There is a zero mode localized on the visible brane and the masses of Kaluza-Klein excitations are naturally of order the weak-scale. We make connection to neutrino phenomenology. We also demonstrate that, contrary to previous claims, a bulk singlet scalar field cannot provide a successful mechanism to generate a suppressed Majorana mass for Standard Model neutrinos localized on the visible brane. 
Klaas Brakke and Elisabetta Pallante