Can an unbroken flavour symmetry provide an approximate description of lepton masses and mixing?

Understand

We provide a complete answer to the following question: what are the flavour groups and representations providing, in the symmetric limit, an approximate description of lepton masses and mixings? We assume that neutrinos masses are described by the Weinberg operator.

• We show that the pattern of lepton masses and mixings only depends on the dimension, type (real, pseudoreal, complex), and equivalence of the irreducible components of the flavour representation, and we find only six viable cases.
• In all cases the neutrinos are either anarchical or have an inverted hierarchical spectrum.
• In the context of SU(5) unification, only the anarchical option is allowed.