Two topics in multiparticle dyamics at LEP: multiplicity in b-bbar events and screwiness at the end of the QCD cascade

TL;DR

This paper reports measurements of charged particle multiplicity in b-bbar events at LEP, confirming QCD predictions, and finds no evidence for the helix ordering of soft gluons at the end of QCD cascades.

Contribution

It provides experimental validation of QCD predictions for b-bbar multiplicity and tests a theoretical model of gluon helix ordering, finding no supporting evidence.

Findings

Charged particle multiplicity in b-bbar events agrees with QCD predictions.

No evidence found for gluon helix ordering at the end of QCD cascades.

Multiplicity measurements challenge models assuming mass-independent decay multiplicity.

Abstract

This talk deals with two topics in multiparticle dynamics investigated by means of the DELPHI detector at LEP. Related to the first topic, we have used the data collected at 183 GeV to measure the average charged particle multiplicity in $e^+e^- \to b\bar{b}$ events. The result is remarkably in agreement with QCD predictions, while it is more than two standard deviations larger than calculations assuming that the multiplicity accompanying the decay of a heavy quark is independent of the mass of the quark itself. The second topic deals with a recent theoretical model by Andersson et al., in which soft gluons order themselves in the form of a helix at the end of the QCD cascades. In our data at the Z peak, we have found no evidence for such an effect.