Measurements of $D^0$ production and of decay branching fractions in neutrino nucleon scattering

During the years 1994-97, the emulsion target of the CHORUS detector was exposed to the wide-band neutrino beam of the CERN SPS of 27 GeV average neutrino energy. In total about 100 000 charged-current neutrino interactions were located in the nuclear emulsion target and fully reconstructed. From this sample of events which was based on the data acquired by new automatic scanning systems, 1048 charged-current interactions with a D^0 in the final state were selected by a pattern recognition program and confirmed as neutral-particle decays through visual inspection. The ratio of decay branching fractions of the D^0 into four-charged particles to two charged particles was measured to B(D^0 -> V4) / B(D^0 -> V2) = 0.207+- 0.004. The inclusive measurement of the observed production rate of the D^0 with a decay into four charged prongs in combination with external measurements of this topological branching ratio was used to determine the total D^0 production rate by neutrinos without additional assumption on the branching fractions. The value of this rate relative to the charged-current cross-section was found to be \sigma(D^0) / \sigma(CC) = 0.0269 +- 0.0018 +- 0.0013. In addition, the same normalization method was used to deduce the inclusive topological decay rate into final states with neutral particles only. A value of 0.218 +- 0.049 +- 0.036 was found for this branching fraction. From an observed number of three charged six-prongs events the branching ratio into six charged particles was determined to be (1.2^1.3_-0.9 +- 0.2) x 10^-3. A measurement of the energy dependence of the D^0 production by neutrinos relative to the total charged-current cross-section is also reported. This measurement was used to deduce for m_c, the effective charm-quark mass, a value of (1.42 +- 0.08) GeV/c^2.