Final results of the OPERA experiment on $\nu_\tau$ appearance in the CNGS beam

TL;DR

The OPERA experiment's final analysis confirms $ u_ au$ appearance in the CNGS beam with high significance, measuring oscillation parameters, $ u_ au$ cross-section, and observing $ u_ au$ lepton number directly.

Contribution

This paper presents the first direct observation of $ u_ au$ lepton number and measures $ u_ au$ cross-section with minimal contamination, using a larger data sample and improved analysis.

Findings

Confirmed $ u_ au$ appearance with 6.1 sigma significance.

Measured $ u_ au$ CC cross-section with negligible contamination.

Observed $ u_ au$ lepton number directly.

Abstract

The OPERA experiment was designed to study $\nu_\mu\to\nu_\tau$ 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 $\nu_\tau$ candidate events, thus reducing the statistical uncertainty in the measurement of the oscillation parameters and of $\nu_\tau$ properties. A multivariate approach for event identification has been applied to the candidate events and the discovery of $\nu_\tau$ appearance is confirmed with an improved significance level of 6.1 $\sigma$. $\Delta m^2_{23}$ has been measured, in appearance mode, with an accuracy of 20%. The measurement of $\nu_\tau$ CC cross-section, for the first time with a negligible contamination from $\bar{\nu}_\tau$, and the first direct observation of the $\nu_\tau$ lepton number are also reported.