Four-Neutrino Mixing

TL;DR

This paper demonstrates that at least four neutrinos are necessary to explain all experimental neutrino oscillation data, and identifies two compatible four-neutrino mass schemes with specific mass spectra.

Contribution

It establishes the minimum number of neutrinos needed and identifies the only two viable four-neutrino schemes consistent with all experimental results.

Findings

At least four neutrinos are required to explain solar, atmospheric, and LSND data.

Only two four-neutrino schemes are compatible with all experiments.

The viable schemes feature two pairs of close-mass neutrinos separated by about 1 eV.

Abstract

It is shown that at least four massive neutrinos are needed in order to accommodate the evidences in favor of neutrino oscillations found in solar and atmospheric neutrino experiments and in the LSND experiment. Among all four-neutrino schemes, only two are compatible with the results of all neutrino oscillation experiments. These two schemes have a mass spectrum composed of two pairs of neutrinos with close masses separated by the "LSND gap" of the order of 1 eV.