Measurement of partial decay widths of the neutral gauge boson Z0 of the weak interaction with the ALEPH detector at the e+e- LEP storage ring

Aim of the present thesis was the experimental determination of the partial decay width of the neutral gauge boson Z0 of the weak interaction. For the analysis the data obtained with the ALEPH detector at the LEP storage ring of the European nuclear research center CERN in the years 1989 and 1990 were available. From the simultaneous numerical fit of both line shapes to a far-reaching model-independent description of the Z0 resonance line important parameters of the standard model of the electroweak interaction could be determined. Assuming the validity of the lepton universality the following properties of the Z0 boson were determined: MZ=(91,183±0,009exp±0,020LEP) GeV, ΓZ=(2493±17) MeV, Γhad=(1751±21) MeV, and Γlept=(83,0±9,2) MeV. The decay width Γuns into experimentally not obervable particles results from this to Γuns=(491.9±19.7) MeV. The number Nν of the fermion families with light neutrinos undetermined in the framework of the theory was calculated to Nν=2.95±0.12exp±0.05theo, from which the existence of a fourth fermion family with ligth neutrinos is to be excluded with a certainty of more than seven standard deviations. The partial decay widths Γhad and Γlept let in the framework of the standard model conclude on a mass mtop of the top quark in the range of mtop=122-43+77 GeV. (orig./HSI)