A search for $\nu_{\mu} \to \nu_{\tau}$ oscillations by CHORUS at CERN

The CHORUS experiment is searching for nu /sub mu / to nu /sub tau / oscillations with a hybrid detector built around a 770 kg nuclear emulsion target. It explores a domain of Delta m/sup 2/>10 eV/sup 2/ and the oscillation probability down to 10/sup -4/. The detector was exposed in 1994-1997 to the wide band nu /sub mu / beam at the CERN SPS. In the analyzed sample containing two thirds of the available statistics, no nu /sub tau / candidates have been found. At present level of the emulsion scanning and measurement efficiency, this results in an upper limit on the oscillation probability of P( nu /sub mu / to nu /sub tau /)<or=4.10/sup -4/ at 90% C.L. A fourfold increase of the sensitivity is expected by the completion of the analysis. (7 refs).