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

TL;DR

This paper analyzes event shape distributions in e+e- collisions across a wide energy range, extracting the strong coupling constant and measuring the QCD beta-function with high precision, while testing for light gluinos.

Contribution

It introduces a combined phenomenological and RGI approach to describe event shape data, enabling precise QCD parameter measurements and new constraints on light gluinos.

Findings

Precise measurement of the QCD beta-function using event shape data.

Determination of the strong coupling constant alpha_s from event shapes.

Exclusion of light gluinos with masses below 5 GeV.

Abstract

Infrared and collinear safe event shape distributions and their mean values are determined in e+e- collisions at centre-of-mass energies between 45 and 202 GeV. 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 +/- 0.32 for the one loop coefficient of the beta-function or, assuming QCD, n_f = 4.75 +/- 0.44 for the number of active flavours. These values agree well with the QCD expectation of beta_0=7.67 and n_f=5. A direct measurement of the full logarithmic energy slope excludes light gluinos with a mass below 5 GeV.