(3+1)-spectrum of neutrino masses: a chance for LSND?

If active to active neutrino transitions are dominant modes of the atmospheric (v(mu) --> v(tau)) and the solar neutrino oscillations (ve --> v(mu)/v(tau)), as is indicated by recent data, the favoured scheme which accommodates the LSND result - the so-called (2 + 2)-scheme - should be discarded. We introduce the parameters eta (atm)(s) and eta (sun)(s) which quantify an involvement of the sterile component in the solar and atmospheric neutrino oscillations. The (2 + 2)-scheme predicts eta (atm)(s) + eta (sun)(s) = 1 and the experimental proof of deviation from this equality will discriminate the scheme. In this connection the (3 + 1)-scheme is revisited in which the fourth (predominantly sterile) neutrino is isolated from a block of three flavour neutrinos by the mass gap Deltam(LSND)(2) similar to (0.4-10) eV(2). We find that in the (3 + 1)-scheme the LSND result can be reconciled with existing bounds on v(e)(-) and v(mu)-disappearance at 95-99% C.L. The generic prediction of the scheme is the v(e) and v(mu)-disappearance probabilities at the level of present experimental bounds. The possibility to strengthen the bound on v(mu)-disappearance in the KEK - front detector experiment is studied. We consider phenomenology of the (3 + l)-scheme, in particular, its implications for the atmospheric neutrinos, neutrinoless double beta decay searches, supernova neutrinos and primordial nucleosynthesis. (C) 2001 Elsevier Science B.V. All rights reserved.