Final Results of the OPERA Experiment on ν_τ Appearance in the CNGS Neutrino Beam

The OPERA experiment was designed to study ν_μ→ν_τ oscillations in the appearance mode in the CERN to Gran Sasso Neutrino beam (CNGS). In this Letter, we report the final analysis of the full data sample collected between 2008 and 2012, corresponding to 17.97×10^19 protons on target. Selection criteria looser than in previous analyses have produced ten ν_τ candidate events, thus reducing the statistical uncertainty in the measurement of the oscillation parameters and of ν_τ properties. A multivariate approach for event identification has been applied to the candidate events and the discovery of ν_τ appearance is confirmed with an improved significance level of 6.1σ. |Δm_32^2| has been measured, in appearance mode, with an accuracy of 20%. The measurement of the ν_τ charged-current cross section, for the first time with a negligible contamination from ν[over ¯]_τ, and the first direct evidence for the ν_τ lepton number are also reported.