Determination of the semi-leptonic branching fraction of neutrino-induced charm hadrons using nuclear emulsion

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 170,000 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 be used for studies of charm production. This work describes the nuclear emulsion technique, with emphasis on the laboratory for automated emulsion scanning at CERN. Automatic emulsion involves computer-steered precision tables, microscope optics with high resolution, an image sensor, custom hardware for real-time image processing and computer infrastructure to store the results, possibly after on-line reconstruction. Among a sample of 56172 neutrino charged current interactions that have so far been analyzed in emulsion, 1442 events have been selected as candidates for charm production. The selection procedure is based on the automatic analysis of emulsion data and reaches a selection purity of 90%. Based on this sample, the average semi-leptonic branching fraction of neutrino-induced charm hadrons has been determined. The result is 10.2 +/- 1.6 (stat.) +/- 1.0 (syst.) %. When combined with the known rate of dimuon events in neutrino interactions, this result leads to a measurement for the absolute value of the CKM matrix element Vcd of 0.219 +/- 0.022.