A Quasi-maximal mixing ansatz for neutrino oscillations

Inspired by the atmospheric multi-GeV neutrino data, we consider neutrino flavor mixing matrices which are maximal in a $2\times 2$ subsector. This condition is a strong restriction: the full matrix including complex phases depends essentially on one parameter. The survival probability is an universal function of $L/E$, independent of generation, in the region of interest for the accelerator and atmospheric experiments. It is possible with the solar neutrino data alone to recover completely the mixing matrix suggested by the atmospheric data. The results are not essentially modified by the MSW effect in the solar data. Consequences for future experiments are considered.