Search for $\nu_\mu\rightarrow\nu_\tau$ oscillation with the OPERA experiment in the CNGS beam

The OPERA neutrino experiment in the underground Gran Sasso Laboratory (LNGS) was designed to perform the first detection of neutrino oscillations in direct appearance mode in the $\nu_\mu\rightarrow\nu_\tau$ channel, the $\nu_\tau$ signature being the identification of the $\tau$-lepton created in its charged current interaction. The hybrid apparatus consists of a large mass emulsion film/lead target complemented by electronic detectors. It is placed in the high energy long-baseline CERN to LNGS neutrino beam (CNGS) 730 km away from the neutrino source. The observation of a first $\nu_\tau$ candidate event was reported in 2010. In this paper, we present the status of the experiment based on the analysis of the data taken during the first two years of operation (2008-2009). The statistical significance of the one event observed so far is then assessed.