Final results of the OPERA experiment on $ν_τ$ appearance in the CNGS beam

Abstract

The OPERA experiment was designed to study $ν_μ\toν_τ$ oscillations in appearance mode in the CNGS neutrino beam. In this letter we report the final analysis of the full data sample collected between 2008 and 2012, corresponding to $17.97\cdot 10^{19}$ 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 6.1 $σ$. $Δm^2_{23}$ has been measured, in appearance mode, with an accuracy of 20%. The measurement of $ν_τ$ CC cross-section, for the first time with a negligible contamination from $\barν_τ$, and the first direct observation of the $ν_τ$ lepton number are also reported.

7 pages, 3 figures