Search for ν_μ→ν_τ oscillation

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During the years 1994-1997,the emulsion target of the CHORUS detector was exposed to the wide band neutrino Beam from the CERN-SPS. About 170000 neutrino interactions were successfully located in the emulsion. Improvements in the automatic emulsion scanning systems and application of different criteria allowed the sample of located events to used for studies of charm production. We present a measurement of the production rate of D0 measons based on a sample of 25693 located nyu・mu charged・current(CC)interactions analyzed sofar. After reconstruction of the event topology in the vertex region,283 D0 decays were observed with an estimated background of 9.2K0 and Λ decays. The ratio of cross・section of D0 production and nyu・mu CC interactions is found to be(l.99+・9.13(stat.)+・0.17(syst.))^*10・2 at 27GeV average nyu・mu energy.