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 ye
ars, 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.
The OPERA neutrino experiment is designed to perform the first observation of neutrino oscillations in direct appearance mode in the $ u_mu to u_tau$ channel, via the detection of the $tau$-leptons created in charged current $ u_tau$ interactions. T
he detector, located in the underground Gran Sasso Laboratory, consists of an emulsion/lead target with an average mass of about 1.2 kt, complemented by electronic detectors. It is exposed to the CERN Neutrinos to Gran Sasso beam, with a baseline of 730 km and a mean energy of 17 GeV. The observation of the first $ u_tau$ candidate event and the analysis of the 2008-2009 neutrino sample have been reported in previous publications. This work describes substantial improvements in the analysis and in the evaluation of the detection efficiencies and backgrounds using new simulation tools. The analysis is extended to a sub-sample of 2010 and 2011 data, resulting from an electronic detector-based pre-selection, in which an additional $ u_tau$ candidate has been observed. The significance of the two events in terms of a $ u_mu to u_tau$ oscillation signal is of 2.40 $sigma$.
