A study of the energy evolution of event shape distributions and their means with the DELPHI detector at LEP

Abstract.

Infrared and collinear safe event shape distributions and their mean values are determined in ${\mathrm{e^+e^-}}$ collisions at centre-of-mass energies between 45 and 202GeV. A phenomenological analysis based on power correction models including hadron mass effects for both differential distributions and mean values is presented. Using power corrections, $\alpha_s$ is extracted from the mean values and shapes. In an alternative approach, renormalisation group invariance (RGI) is used as an explicit constraint, leading to a consistent description of mean values without the need for sizeable power corrections. The QCD $\beta$-function is precisely measured using this approach. From the DELPHI data on Thrust, including data from low energy experiments, one finds

$$\beta_0 = 7.86 \pm 0.32$$

for the one loop coefficient of the $\beta$-function or, assuming QCD,

$$n_{\mathrm{f}} = 4.75 \pm 0.44 $$

for the number of active flavours. These values agree well with the QCD expectation of $\beta_0=7.67$ and $n_{\mathrm{f}}=5$. A direct measurement of the full logarithmic energy slope excludes light gluinos with a mass below 5GeV.