Search for $\nu_{\mu} \to \nu_{\tau}$ oscillations in appearance mode in the OPERA experiment

The OPERA experiment in the underground Gran Sasso Laboratory (LNGS) has been designed to perform the first detection of neutrino oscillations in direct appearance mode in the muon neutrino to tau neutrino channel. The detector is hybrid, being made of an emulsion/lead target and of electronic detectors. It is placed in the CNGS neutrino beam 730 km away from the neutrino source. Runs with CNGS neutrinos were successfully carried out in 2008, 2009, and 2010. After a brief description of the beam and the experimental setup, we report on event analysis of a sample of events corresponding to 1.89\times 10^{19} p.o.t. in the CERN CNGS \nu_{\mu} beam that yielded the observation of a first candidate \nu_{\tau} CC interaction. The topology and kinematics of this candidate event are described in detail. The background sources are explained and the significance of the candidate is assessed.