A new class of models with pseudo Nambu-Goldstone bosons is constructed using a non-Abelian symmetry in the right-handed Majorana neutrino sector of seesaw neutrino mass models. The phase structure of these models is examined both at zero and nonzero temperatures, with particular emphasis on their phase transition characteristics. We find that the vacuum manifold of these models exhibits a rich structure in terms of possible topological defects, and we argue that these models may have applications to late-time phase transition theories of structure formation.

• Received 24 April 1992

DOI:https://doi.org/10.1103/PhysRevD.47.421