  This thesis gives a comprehensive analysis of hadron production in
$e^{+}e^{-}$ collisions at different center of mass energies in the framework
of the Statistical Model of the hadron resonance gas. The model used for the
analysis is formulated in the canonical ensemble with exact conservation of
five quantum numbers. The corresponding canonical partition function in quantum
statistics is derived and the canonical factor as the striking feature of the
canonical framework will be investigated in detail. The parameters of the
underlying model were determined using a fit to the average multiplicities of
the latest measurements at $\sqrt{s}$=10, 29-35, 91 and 130-200 GeV. The
measurements are compared to the pure thermal production like in heavy ion
collisions as well as to the production in a second scenario accounting for
hard collisions and the resulting enhancement of heavy quark. The results
demonstrate that, within the accuracy of the experiments, none of the data sets
is satisfactorily described with this approach, calling into question the
notion that particle production in $e^{+}e^{-}$ collisions is thermal in
origin. Nevertheless it is observed that the charm and bottom particle yields
produced in hard collisions can be explained by large canonical enhancements of
such particles in heavy jets, almost independent of the model parameters.

Nuclear Theory