On the origin of neutrino masses

We discuss the simplest mechanisms for generating neutrino masses at tree level and one loop level. We find a significant number of new possibilities where one can generate neutrino masses at the one-loop level by adding only two new types of representations. These models have renormalizable interactions that automatically conserve baryon number. Adding to the minimal standard model a scalar color octet with SU(3)⊗SU(2)⊗U(1) quantum numbers, (8,2,1/2), and a fermionic color octet in the fundamental or adjoint representation of SU(2), one can generate neutrino masses in agreement with experiment. Signals at the LHC and constraints from flavor violation are briefly discussed.