The irresistible call of $\tau=i$

Abstract

We analyze a large set of modular invariant models of lepton masses and mixing angles, pointing out that many of them prefer to live close to the self-dual point $\tau=i$. We show that in the vicinity of this point a universal behavior naturally emerges, independently from details of the theory such as the finite modular group acting on the lepton multiplets, the weights of the matter multiplets and even the form of the kinetic terms, which are not required to be neither minimal nor flavour blind. The neutrino mass spectrum is normally ordered and universal relations describe the scaling of the physical observables in terms of the parameter $|\tau-i|$.