Evidence for<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>ν</mml:mi><mml:mi>μ</mml:mi></mml:msub><mml:mo>→</mml:mo><mml:msub><mml:mi>ν</mml:mi><mml:mi>τ</mml:mi></mml:msub></mml:math>appearance in the CNGS neutrino beam with the OPERA experiment

The OPERA experiment is designed to search for ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ oscillations in appearance mode, i.e., through the direct observation of the $\ensuremath{\tau}$ lepton in ${\ensuremath{\nu}}_{\ensuremath{\tau}}$-charged current interactions. The experiment has taken data for five years, since 2008, with the CERN Neutrino to Gran Sasso beam. Previously, two ${\ensuremath{\nu}}_{\ensuremath{\tau}}$ candidates with a $\ensuremath{\tau}$ decaying into hadrons were observed in a subsample of data of the 2008--2011 runs. Here we report the observation of a third ${\ensuremath{\nu}}_{\ensuremath{\tau}}$ candidate in the ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay channel coming from the analysis of a subsample of the 2012 run. Taking into account the estimated background, the absence of ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ oscillations is excluded at the $3.4\text{ }\ensuremath{\sigma}$ level.