Experimental properties of gluon and quark jets from a point source

TL;DR

This paper investigates the properties of gluon and quark jets from a point source in Z0 decays, measuring particle distributions and multiplicities, and tests QCD predictions and color reconnection models.

Contribution

It provides detailed experimental measurements of gluon and quark jet properties, and critically tests the color reconnection model in Monte Carlo simulations.

Findings

Gluon to quark jet multiplicity ratio is 2.29, matching QCD predictions.

Disfavored the color reconnection model in Ariadne Monte Carlo.

Measured distributions of rapidity, energy, and transverse momentum for charged particles.

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.