Novel relations between lepton and quark mixings

A novel relation is found which gives the 2-3 lepton mixing angle θμτ in terms of quark masses and CKM mixing:tanθμτ=(mb/ms)Vcb.This relation, which is remarkably in good agreement with the quark data and the large lepton mixing recently observed, is a kind of SO(10) GUT relation similar to the celebrated bottom-tau mass ratio in SU(5). The GUT models in which this relation hold should have a `twisted SO(10)ʹ structure between the second and third generations, in order to really explain the large lepton mixing, or equivalently, the right-hand side of the relation.