A review of μ-τ flavor symmetry in neutrino physics

Behind the observed pattern of lepton flavor mixing is a partial or approximate mu-tau flavor symmetry-a milestone on our road to the true origin of neutrino masses and flavor structures. In this review article we first describe the features of mu-tau permutation and reflection symmetries, and then explore their various consequences on model building and neutrino phenomenology. We pay particular attention to soft mu-tau symmetry breaking, which is crucial for our deeper understanding of the fine effects of flavor mixing and CP violation.