The observed quark hierarchies suggest a simple family symmetry. Generalized to leptons through grand-unified quantum numbers, it produces a neutrino mixing matrix with order-1 ν_μ-ν_τ mixing, and order- λ³ νₑ-ν_μ and νₑ-ν_τ mixings. The intrafamily hierarchy and observed neutrino mass differences together require this symmetry to be anomalous, suggesting through the Green-Schwarz mechanism a string or M-theory origin for the symmetry.
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Elwood et al. (1998) studied this question.
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