A Modular $SU(5)$ Littlest Seesaw

Abstract

We extend the littlest modular seesaw to a Grand Unified scenario based on $SU(5)$ endowed with three modular $S_4$ symmetries. We leverage symmetry protected zeroes in the leptonic and down quark sectors to suppress deviations to the littlest modular seesaw predictions, but not contributions to the quark mixing. The model is supplemented by two weighton fields, such that the hierarchical nature of the charged-lepton masses, as well as the quark masses and mixing, stem from the content and symmetries of the model, rather than a hierarchical nature of the Yukawa coefficients.