Fermion masses, critical behavior and universality

Abstract

We look for signals of critical behavior in the Yukawa sector. By reviewing a set of models for the fermion masses, we select those where a symmetry-breaking order parameter sits at a transition point between a disordered phase and an ordered one. Many models based on ordinary flavor symmetries are formulated in terms of small corrections to a symmetric limit, which can hardly be interpreted unambiguously as a sign of near-criticality. Different is the case of nonlinearly realized flavor symmetries when the system is always in the broken phase. By inspecting a large number of modular and CP invariant models of lepton masses, we find that most of them cluster around the fixed point $τ= i$, where the system enjoys enhanced symmetry. Since a priori all values of the modulus $τ$ are equally acceptable to describe the fermion spectrum, we regard this preference as a hint of near-criticality. We analyze in detail these models in the vicinity of all fixed points, showing that only one possibility provides a good description of neutrino masses and mixing angles. Near the fixed points the models exhibit a universal behavior. Mass ratios and mixing angles scale with appropriate powers of the order parameter, independently of the details of the theory, a feature reminiscent of systems belonging to the same universality class in second-order phase transitions. The observations of this work are inspired by the role near-criticality might play in solving the naturalness problem and are motivated by the fascinating possibility that most of the free parameters of the Standard Model could find a common explanation.