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

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