Tri-bimaximal mixing and the neutrino oscillation data

Following recent results from the SNO solar neutrino experiment and the K2K long-baseline neutrino experiment, the combined existing data on neutrino oscillations now point strongly to a specific form for the lepton mixing matrix, with effective bimaximal mixing of νμ and ντ at the atmospheric scale and effective trimaximal mixing for νe with νμ and ντ at the solar scale (hence ‘tri-bimaximal’ mixing). We give simple mass-matrices leading to tri-bimaximal mixing, and discuss its relation to the Fritzsch–Xing democratic ansatz.