Residual Z2 x Z2 symmetries and lepton mixing

We consider two novel scenarios of residual symmetries of the lepton mass matrices. Firstly we assume a Z2 x Z2 symmetry G_ell for the charged-lepton mass matrix and a Z2 symmetry G_nu for the light neutrino mass matrix. With this setting, the moduli of the elements of one column of the lepton mixing matrix are fixed up to a reordering. One may interchange the roles of G_ell and G_nu in this scenario, thereby constraining a row, instead of a column, of the mixing matrix. Secondly we assume a residual symmetry group G_ell \cong Zm (m>2) which is generated by a matrix with a doubly-degenerate eigenvalue. Then, with G_nu \cong Z2 x Z2 the moduli of the elements of a row of the lepton mixing matrix get fixed. Using the library of small groups we have performed a search for groups which may embed G_ell and G_nu in each of these two scenarios. We have found only two phenomenologically viable possibilities, one of them constraining a column and the other one a row of the mixing matrix.

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