Misure di fisica elettrodebole con il quark charm al picco della Z

In this thesis charm electroweak measurements at the Z peak with the full statistics collected by ALEPH are presented. The forward-backward asymmetry of the charm quark and the fraction of hadronic Z decays into charm quark pairs, R_c, are measured using D*+ (D*-) mesons as a signature of the charm quark (anti-quark) production. The branching ratio of D0 --> K-pi+, which plays an important role in heavy flavour physics and is here used to extract R_c, is also measured using D* mesons. The kinematics of the decay D*+ --> D0 pi+, followed by the D0 decay in K-pi+, K-pi+pi0 or K-pi+pi+pi-, allows the reconstruction of the decay chain to be performed with high purity. The beauty and charm contribution to the sample is determined by means of a lifetime-mass tag. The asymmetry is extracted from a likelihood fit to the angular distribution of the thrust axis in the selected D* sample. The branching ratio D0 --> K-pi+ is measured by compairing the rate for the reconstructed D*+ --> D0 pi+, D0 --> K-pi+ decay chain and the rate for inclusive soft pion production at low transverse momentum with respect to the nearest jet, which is a clear signature of the D*+ --> D0 pi+ decay. The fraction of hadronic Z decays into charm quark pairs is measured using a double-tag technique, where a charm quark is tagged from the D*'s fully reconstructed and the other one from the soft pion.