Particle multiplicity of unbiased gluon jets from e+e- three-jet events

TL;DR

This study measures and compares the charged particle multiplicities in gluon and quark jets from e+e- collisions, extracting unbiased gluon jet data and validating QCD predictions about their multiplicity ratios and color factors.

Contribution

It provides the first extraction of unbiased gluon jet multiplicities over a range of energies using theoretical bias corrections, and compares these to quark jets to test QCD predictions.

Findings

Gluon to quark jet multiplicity ratio at 30 GeV is 1.422 +/- 0.051.

Measured ratios of slopes and curvatures support QCD predictions.

Effective CA/CF ratio is 2.23 +/- 0.14, consistent with QCD value of 2.25.

Abstract

The charged particle multiplicities of two- and three-jet events from the reaction e+e- -> Z0 -> hadrons are measured for Z0 decays to light quark (uds) flavors. Using recent theoretical expressions to account for biases from event selection, results corresponding to unbiased gluon jets are extracted over a range of jet energies from about 11 to 30 GeV. We find consistency between these results and direct measurements of unbiased gluon jet multiplicity from upsilon and Z0 decays. The unbiased gluon jet data including the direct measurements are compared to corresponding results for quark jets. We perform fits based on analytic expressions for particle multiplicity in jets to determine the ratio r = Ng/Nq of multiplicities between gluon and quark jets as a function of energy. We also determine the ratio of slopes, r(1) = (dNg/dy)/(dNq/dy), and of curvatures, r(2) = (d2Ng/dy2)/(d2Nq/dy2), where y specifies the energy scale. At 30 GeV, we find r = 1.422 +/- 0.051, r(1) = 1.761 +/- 0.071 and r(2) = 1.98 +/- 0.13, where the uncertainties are the statistical and systematic terms added in quadrature. These results are in general agreement with theoretical predictions. In addition, we use the measurements of the energy dependence of Ng and Nq to determine an effective value of the ratio of QCD color factors, CA/CF. Our result, CA/CF = 2.23 +/- 0.14 (total), is consistent with the QCD value of 2.25.