Four-Neutrino Oscillations

TL;DR

This paper demonstrates that at least four neutrinos are necessary to explain all experimental evidence for neutrino oscillations, identifying two compatible mass schemes with specific implications for long-baseline experiments.

Contribution

It identifies the minimal four-neutrino framework compatible with all current oscillation data and characterizes the properties of these schemes.

Findings

Only two four-neutrino schemes fit all data.

In these schemes, certain transition probabilities are suppressed.

CP violation effects are small in long-baseline experiments.

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, with a mass spectrum composed of two pairs of neutrinos with close masses separated by the "LSND gap" of the order of 1 eV, are compatible with the results of all neutrino oscillation experiments. In these two schemes the probability of nu_e transitions into other states, the probability of nu_mu->nu_e transitions and the size of CP violation effects in nu_mu->nu_e transitions are suppressed in long-baseline experiments.