Observation of tau neutrino appearance in the CNGS beam with the OPERA experiment

Abstract

The OPERA experiment is searching for νμ ν τ oscillations in appearance mode, i.e., via the direct detection of \tau leptons in u -\tau charged-current interactions. The evidence of u -\mu \rightarrow u -\tau appearance has been previously reported with three u -\tau candidate events using a sub-sample of data from the 2008-2012 runs. We report here a fourth u -\tau candidate event, with the \tau decaying into a hadron, found after adding the 2012 run events without any muon in the final state to the data sample. Given the number of analyzed events and the low background, u -\mu \rightarrow u -\tau oscillations are established with a significance of 4.2\sigma.