Energy dependence of mean multiplicities in gluon and quark jets at the next-to-next-to-next-to-leading order

TL;DR

This paper presents high-order perturbative QCD predictions for how mean multiplicities in gluon and quark jets depend on energy, comparing them with experimental data to evaluate their accuracy.

Contribution

It provides the first next-to-next-to-next-to-leading order (3NLO) calculations for jet multiplicities and analyzes their agreement with experimental results.

Findings

3NLO correction to gluon jet multiplicity is small and fits data well.

3NLO correction to quark jet multiplicity is large and does not fit data.

NLO success for quark jets is due to near equivalence with gluon expressions at that order.

Abstract

Analytic predictions for the energy dependence of the mean multiplicities in gluon and quark jets are presented at the next-to-next-to-next-to-leading order (3NLO) of perturbative QCD and are compared to experiment. The 3NLO correction to the gluon jet multiplicity is found to be small. The corresponding theoretical expression provides a good description of available gluon jet measurements. In contrast, the 3NLO correction to the quark jet multiplicity is large and the theoretical expression does not describe the data accurately. It is shown that the well known success of the next-to-leading order (NLO) approximation in describing the energy evolution of quark jet multiplicity can be attributed to the equivalence of the quark and gluon expressions at NLO to within a constant factor, and to almost constant contributions from higher order terms to the gluon jet result.