Mu-tau symmetry and the Littlest Seesaw

Abstract

Motivated by the latest neutrino oscillation data which is consistent with maximal atmospheric mixing and maximal leptonic CP violation, we review various results in $\mu\tau$ symmetry, then include several new observations and clarifications, including identifying a new general form of neutrino mass matrix with $\mu\tau$ symmetry. We then apply the new results to the neutrino mass matrix associated with the Littlest Seesaw model, and show that it approximately satisfies the new general form with $\mu\tau$ symmetry, and that this is responsible for its approximate predictions of maximal atmospheric mixing and maximal CP violation in the lepton sector.