Orbifold modular GUT of flavor

We discuss an SU(5) grand unified theory based on the 10d orbifold (T2)3/(Z4×Z2) plus three modular S4 groups with moduli at respective fixed points i,i+2,ω=e2iπ/3. The resulting model has hierarchical quark and charged-lepton mass matrices, arising from a double weighton mechanism, and reproduces the highly predictive littlest seesaw mechanism in the neutrino sector. The down-quark mass matrix has an upper triangular form, contributing to Cabibbo-Kobayashi-Maskawa mixing, while the charged-lepton mass matrix has a lower-triangular form with suppressed contributions to Pontecorvo-Maki-Nakagawa-Sakata mixing. The orbifold yields successful SU(5) breaking with doublet-triplet splitting of the Higgs multiplets. Published by the American Physical Society 2024