The OPERA experiment was designed to search for $ u_{mu} rightarrow u_{tau}$ oscillations in appearance mode, i.e. by detecting the $tau$-leptons produced in charged current $ u_{tau}$ interactions. The experiment took data from 2008 to 2012 in the CERN Neutrinos to Gran Sasso beam. The observation of $ u_{mu} rightarrow u_{tau}$ appearance, achieved with four candidate events in a sub-sample of the data, was previously reported. In this paper, a fifth $ u_{tau}$ candidate event, found in an enlarged data sample, is described. Together with a further reduction of the expected background, the candidate events detected so far allow assessing the discovery of $ u_{mu}rightarrow u_{tau}$ oscillations in appearance mode with a significance larger than 5 $sigma$.
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