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

The OPERA experiment is searching for νμ→ντ oscillations in appearance mode, i.e., via the direct detection of τ leptons in ντ charged-current interactions. The evidence of νμ→ντ appearance has been previously reported with three ντ candidate events using a sub-sample of data from the 2008–2012 runs. We report here a fourth ντ candidate event, with the τ 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, νμ→ντ oscillations are established with a significance of 4.2σ.