Residual flavour (anti)symmetries at the modular self-dual point and constraints on neutrino masses and mixing

Abstract: We explore the implications of symmetries that remain unbroken at the self-dual point $\tau=i$ in modular invariant theories. Assuming that (a) the three generations of lepton doublets transform as an irreducible representation of a finite modular group $\Gamma_N$, and (b) the light neutrino masses arise from the Weinberg operator and are in modular form, we demonstrate that this setup yields a unique residual flavor symmetry or antisymmetry for the neutrinos, depending on the modular weight. In the antisymmetric case, one neutrino is always massless, and the other two can be degenerate if the mass matrix is real. These findings are independent of the level $N$. If the charged leptons are arranged to exhibit an appropriate residual symmetry from the same $\Gamma_N$, they determine a column of the leptonic mixing matrix, leading to specific correlations between the mixing angles and the Dirac CP phase. The presence of residual (anti)symmetries enables the application of standard flavor symmetry techniques to derive these predictions, and we scan all possible $\Gamma_N$ satisfying condition (a). Most solutions yield ${\cal O}(1)$ entries in the fixed column, favouring relatively large lepton mixing.