Discrete Gauge Symmetries in Axionic Extensions of the SSM
  We examine discrete gauge symmetries in axionic extensions of the SSM which provide a solution of the $\mu$-problem. Automatic-PQ symmetry and proton stability are shown to be guaranteed by certain discrete symmetries. Focusing on the L-violating discrete symmetries we discuss two sources of neutrino masses and their relevance for the solar neutrino problem. 
E. J. Chun and A. Lukas