Four-neutrino MS^2 mixing

We present a simple scheme of mixing of four neutrinos that can accommodate the results of all neutrino oscillation experiments, the observed abundances of primordial elements and the current upper bound for the effective Majorana neutrino mass in neutrinoless double-beta decay (assuming that massive neutrinos are Majorana particles). The scheme has maximal mixing in the nu_mu,nu_tau-nu_3,nu_4 sector and small mixings in the nu_e,nu_s-nu_1,nu_2 and nu_e,nu_mu-nu_1,nu_3 (or nu_e,nu_mu-nu_1,nu_4) sectors (MS^2). We discuss the implications of this scheme for short and long baseline oscillation experiments and for neutrinoless double-beta decay and tritium beta-decay experiments.