The OPERA experiment: a direct search of the $\nu_\mu \longrightarrow \nu_\tau$ oscillations

The aim of the OPERA experiment is to search for the appearance of the tau neutrino in the quasi pure muon neutrino beam produced at CERN (CNGS). The detector, installed in the Gran Sasso underground laboratory 730 km away from CERN, consists of a lead/emulsion target complemented with electronic detectors. After a short pilot run in 2007, a first physics run took place from June to November 2008. The second physics run started in June 2009. At present a total (2008+2009) of 4.2 10$^{19}$ protons on target were delivered by the CNGS, producing more than 25,000 events in time coincidence in the OPERA detector. Among them \~4000 events occured in the target of the detector. In this paper the detector and the analysis strategy are described and the status of the analysis of the 2008 and 2009 runs is discussed.