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