Inclusive momentum distributions of charged particles are measured in dijet events. Events were produced at the AMY detector with a centre of mass energy of 60 ${\rm GeV}$. Our results were compared, on the one hand to those obtained from other $e^+ e^-$, $ep$ as well as CDF data, and on the other hand to the perturbative QCD calculations carried out in the framework of the modified leading log approximation (MLLA) and assuming local parton--hadron duality (LPHD). A fit of the shape of the distributions yields $\scr Q_{eff} = 263 \pm 13 {\rm MeV}$ for the AMY data. In addition, a fit to the evolution of the peak position with dijet mass using all data from different experiments gives $\scr Q_{eff} = 226 \pm 18 {\rm MeV}$. Next, αs was extracted using the shape of the distribution at the Z0 scale, with a value of 0.118 \pm 0.013. This is consistent, within the statistical errors, with many accurate measurements. We conclude that it is the success of LPHD + MLLA that the extracted value of $\alpha_{s}$ is correct. Possible explanations for all these features will be presented in this paper.

We present the properties of the C-parameter as an event-shape variable. We calculate the coupling constants in the perturbative and also in the non-perturbative parts of the QCD theory, using the dispersive as wellas the shape function models. By fitting the corresponding theoretical predictions to our data, we find $\alpha_{s}(M_{Z^{0}} ) = 0.117 \pm 0.014$ and $\alpha_{0}(\mu_{I} ) = 0.491 \pm 0.043$ for dispersive model and $\alpha_{ s}(M_{Z^{0}} ) = 0.124 \pm 0.015$ and $\lambda_{1} = 1.234 \pm 0.052$ for the shape function model. Our results are consistent with the world average value of $\alpha_{s}(M_{Z^{0}} ) = 0.118 \pm 0.002$. All these features are explained in the main text.