Experimental properties of gluon and quark jets from a point source

TL;DR

This OPAL study defines gluon jets as hemispheres opposite two tagged quark jets, creating a clean proxy for gluon jets from a color-singlet source and enabling direct comparisons to uds quark jets. It reports rapidity, fragmentation, and momentum distributions, along with multiplicity in restricted rapidity regions, and finds a soft-particle multiplicity ratio close to the QCD color-factor prediction $C_A/C_F=2.25$ after accounting for finite-energy and hadronization effects. The measurements of gluon-jet properties largely agree with standard MC models, but color reconnection scenarios in Ariadne (AR-2, AR-3) are strongly disfavored by the data. The results provide a direct, high-precision test of QCD jet dynamics at LEP energies and place constraints on non-perturbative reconnection effects relevant for precision electroweak measurements.

Abstract

Gluon jets are identified in hadronic Z0 decays as all the particles in a hemisphere opposite to a hemisphere containing two tagged quark jets. Gluon jets defined in this manner are equivalent to gluon jets produced from a color singlet point source and thus correspond to the definition employed for most theoretical calculations. In a separate stage of the analysis, we select quark jets in a manner to correspond to calculations, as the particles in hemispheres of flavor tagged light quark (uds) events. We present the distributions of rapidity, scaled energy, the logarithm of the momentum, and transverse momentum with respect to the jet axes, for charged particles in these gluon and quark jets. We also examine the charged particle multiplicity distributions of the jets in restricted intervals of rapidity. For soft particles at large transverse momentum, we observe the charged particle multiplicity ratio of gluon to quark jets to be 2.29 +- 0.09 +- 0.15 in agreement with the prediction that this ratio should approximately equal the ratio of QCD color factors, CA/CF = 2.25. The intervals used to define soft particles and large transverse momentum for this result, p<4 GeV/c and 0.8<p_t<3.0 GeV/c, are motivated by the predictions of the Herwig Monte Carlo multihadronic event generator. Additionally, our gluon jet data allow a sensitive test of the phenomenon of non-leading QCD terms known as color reconnection. We test the model of color reconnection implemented in the Ariadne Monte Carlo multihadronic event generator and find it to be disfavored by our data.

Figures (15)

• Figure 1: Event types pertinent to this analysis. The dashed vertical lines represent hemisphere boundaries, defined in our study by the plane perpendicular to the thrust axis, while the large solid dots represent a color singlet point source. (a) gg production. (b) e$^+$e$^-$$\rightarrow\,$q$_{\mathrm{tag}} \overline{\mathrm{q}}_{\mathrm{tag}}$g$_{\,\mathrm{incl.}}$: The quark jets q$_{\mathrm{tag}}$ and $\overline{\mathrm{q}}_{\mathrm{tag}}$ are tagged b jets defined using a jet algorithm and are used only as a tool to identify the g$_{\,\mathrm{incl.}}$ jet hemispheres. The g$_{\,\mathrm{incl.}}$ jet hemispheres provide the gluon jet sample for our study. The g$_{\,\mathrm{incl.}}$ jets yield virtually the same results for the experimental observables in our study as the hemispheres of gg events shown in part (a). (c) e$^+$e$^-$$\rightarrow\,$q$\overline{\mathrm{q}}$, with q a light (uds) quark: Hemispheres in these events provide the quark jet sample for our study.
• Figure 2: The prediction of the Herwig parton shower Monte Carlo event generator for the charged particle (a) rapidity and (b) $x_{E}$$\,$=$\,$$E/E_{\,\mathrm{jet}}$ distributions of g$_{\,\mathrm{incl.}}$ gluon jets from e$^+$e$^-$ annihilations, in comparison to the Herwig predictions for gg and uds event hemispheres. The jet energies are 40 GeV, corresponding to a c.m. energy of 91.2 GeV for the generation of the e$^+$e$^-$$\rightarrow\,$q$\overline{\mathrm{q}}$$\,$g$_{\,\mathrm{incl.}}$ events.
• Figure 3: The prediction of the Herwig parton shower Monte Carlo event generator for the charged particle (a) $\ln\,(p)$ and (b) $p_{\mathrm{T}}\,(p\!<\!4~{\mathrm{GeV}}/c)$ distributions of g$_{\,\mathrm{incl.}}$ gluon jets from e$^+$e$^-$ annihilations, in comparison to the Herwig predictions for gg and uds event hemispheres. The jet energies are 40 GeV, corresponding to a c.m. energy of 91.2 GeV for the generation of the e$^+$e$^-$$\rightarrow\,$q$\overline{\mathrm{q}}$$\,$g$_{\,\mathrm{incl.}}$ events.
• Figure 4: The prediction of the Herwig parton shower Monte Carlo event generator for the charged particle multiplicity distributions of g$_{\,\mathrm{incl.}}$ gluon jets from e$^+$e$^-$ annihilations, in the rapidity intervals (a) $|y|$$\,\leq\,$2 and (b) $|y|$$\,\leq\,$1, in comparison to the Herwig predictions for gg and uds event hemispheres. The jet energies are 40~GeV, corresponding to a c.m. energy of 91.2~GeV for the generation of the e$^+$e$^-$$\rightarrow\,$q$\overline{\mathrm{q}}$$\,$g$_{\,\mathrm{incl.}}$ events.
• Figure 5: (a) Corrected distributions of charged particle rapidity, $y$, for 40.1 GeV g$_{\,\mathrm{incl.}}$ gluon jets and 45.6 GeV uds quark jets. (b) The ratio of the gluon to quark jet rapidity distributions for 40.1 GeV jets. The total uncertainties are shown by vertical lines. The experimental statistical uncertainties are indicated by small horizontal bars. (The uncertainties are too small to be seen for the uds jets.) The predictions of various parton shower Monte Carlo event generators are also shown. These data are tabulated in Table \ref{['tab-rapidity']}.