Search for nu_mu --> nu_tau oscillations in the OPERA experiment

The OPERA neutrino experiment in the underground Gran Sasso Laboratory (LNGS) was designed to perform the first detection of neutrino oscillations in the nu_mu --> nu_tau channel in appearance mode. The nu_tau signature is provided by the direct observation of the tau lepton produced in nu_tau interaction and decaying with a flight length of few hundred microns. The detection of both the tau production and decay vertices is achieved thanks to the micrometric accuracy of nuclear emulsions . The apparatus is hybrid: it consists of a 1.25 kton emulsion/lead target complemented by electronic detectors. The detector is placed in the high energy long-baseline CERN to LNGS neutrino beam (CNGS), 730 km away from the neutrino source. Three candidates have been observed in the current analysed statistics, satisfying the kinematical selection criteria. Their topology and kinematics has been fully studied and described here in detail. The efficiency of the nu_tau event selection has been evaluated for the different tau decay channels. The background yield has been evaluated and the kinematical selection optimized in order to have a better discrimination. The statistical significance of the nu_mu --> nu_tau observation is 3.5 sigma.