Lepton mixing from the $Δ(96)$ Modular Littlest Seesaw

Abstract

We perform the first comprehensive and model independent study of Modular Littlest Seesaw models based on the finite modular group $Δ(96)$. We construct the vector-valued modular forms (VVMFs) for all irreducible representations of modular $Δ(96)$, classify the inequivalent symmetry-preserving fixed points, and derive the corresponding alignments of the low-weight and next-to-lowest-weight triplet VVMFs. These results allow an exhaustive scan over the residual symmetries in the charged lepton, atmospheric neutrino, and solar neutrino sectors. We identify 35 phenomenologically viable and inequivalent breaking patterns, including 21 with normal ordering and 14 with inverted ordering. The resulting Dirac neutrino mass matrices go beyond the conventional CSD$(n)$ structure, yielding new fixed PMNS columns and novel correlations among the lepton mixing parameters beyond the TM$_1$ paradigm. The viable models are highly predictive, giving narrow ranges for neutrino masses, mixing parameters and CP phases, and can be stringently tested by upcoming experiments such as JUNO, DUNE and T2HK.