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

If active to active neutrino transitions are dominant modes of the atmospheric (νμ→ντ) and the solar neutrino oscillations (νe→νμ/ντ), 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 ηsatm and ηssun which quantify an involvement of the sterile component in the solar and atmospheric neutrino oscillations. The (2+2)-scheme predicts ηsatm+ηssun=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 Δm2LSND∼(0.4–10) eV2. We find that in the (3+1)-scheme the LSND result can be reconciled with existing bounds on νe- and νμ-disappearance at 95–99% C.L. The generic prediction of the scheme is the νe- and νμ-disappearance probabilities at the level of present experimental bounds. The possibility to strengthen the bound on νμ-disappearance in the KEK — front detector experiment is studied. We consider phenomenology of the (3+1)-scheme, in particular, its implications for the atmospheric neutrinos, neutrinoless double beta decay searches, supernova neutrinos and primordial nucleosynthesis.