NEUTRINO MASS IN GRAND UNIFIED SU(15)

We explore various scenarios in which neutrinos can obtain mass in a recently discussed model of grand unification based on the gauge group SU(15). Since SU(15) has only left-handed neutrinos the mass is of Majorana type which necessarily violates lepton-number symmetry. We show that, by introducing the 15-dimensional Higgs-boson multiplet, one can naturally obtain a small neutrino mass through lepton-number violation which already exists at the unification scale.