mu-tau Interchange symmetry and lepton mixing
  We focus on the use of a mu-tau interchange symmetry to explain features of lepton mixing, especially maximal atmospheric neutrino mixing. We review two models which achieve this goal and are based on the seesaw mechanism and on the soft breaking of the family-lepton-number symmetries. We also note that that symmetry may be embedded in a generalized CP symmetry. We show that, in the context of some of our models, arguments of naturalness may be used for explaining the smallness of the mass ratio m_mu / m_tau. 
W. Grimus, L. Lavoura
hep-ph