Charged Particle Multiplicity in Three-Jet Events and Two-Gluon Systems

TL;DR

This paper measures charged particle multiplicities in three-jet events from Z decays, confirming QCD predictions for the colour factor ratio and comparing gluon and quark multiplicities, highlighting the role of non-perturbative effects.

Contribution

It provides a precise measurement of the colour factor ratio C_A/C_F and the multiplicity of two-gluon states, testing QCD predictions against experimental data.

Findings

Measured C_A/C_F = 2.261 with uncertainties, consistent with SU(3) QCD.

Found good agreement for the derivative ratio r^{(1)} with perturbative calculations.

Observed deviations in the multiplicity ratio r, indicating significant non-perturbative effects.

Abstract

The charged particle multiplicity in hadronic three-jet events from Z decays is investigated. The topology dependence of the event multiplicity is found to be well described by a modified leading logarithmic prediction. A parameter fit of the prediction to the data yields a measurement of the colour factor ratio C_A/C_F with the result C_A/C_F = 2.261 +/- 0.014 (stat.) +/- 0.036 (exp.) +/- 0.066 (theo.) in agreement with the SU(3) expectation of QCD. The quark-related contribution to the event multiplicity is subtracted from the three-jet event multiplicity resulting in a measurement of the multiplicity of two-gluon colour-singlet states over a wide energy range. The ratios r=N_{gg}(s)/N_{q qbar}(s) of the gluon and quark multiplicities and r^{(1)}=N_{gg}'(s)/N_{q qbar}'(s) of their derivatives are compared with perturbative calculations. While a good agreement between calculations and data is observed for r^{(1)}, larger deviations are found for r indicating that non-perturbative effects are more important for r than for r^{(1)}.