Average gluon and quark jet multiplicities at higher orders

TL;DR

This paper introduces a new formalism for calculating gluon and quark jet multiplicities in QCD, incorporating higher-order resummations and nonperturbative effects, leading to precise predictions consistent with experimental data.

Contribution

It presents a novel NNLL resummed formalism for jet multiplicities that includes nonperturbative parameters and achieves high-precision fits to data, improving upon previous models.

Findings

Achieves less than 5% uncertainty for scales above 10 GeV.

Extracts the strong coupling constant with high precision, alpha_s^(5)(M_Z)=0.1199 +- 0.0026.

Demonstrates the formalism's effectiveness in solving longstanding QCD problems.

Abstract

We develop a new formalism for computing and including both the perturbative and nonperturbative QCD contributions to the scale evolution of average gluon and quark jet multiplicities. The new method is motivated by recent progress in timelike small-x resummation obtained in the MS-bar factorization scheme. We obtain next-to-next-to-leading-logarithmic (NNLL) resummed expressions, which represent generalizations of previous analytic results. Our expressions depend on two nonperturbative parameters with clear and simple physical interpretations. A global fit of these two quantities to all available experimental data sets that are compatible with regard to the jet algorithms demonstrates by its goodness how our results solve a longstandig problem of QCD. We show that the statistical and theoretical uncertainties both do not exceed 5% for scales above 10 GeV. We finally propose to use the jet multiplicity data as a new way to extract the strong-coupling constant. Including all the available theoretical input within our approach, we obtain alpha_s^(5)(M_Z)=0.1199 +- 0.0026 in the MS-bar scheme in an approximation equivalent to next-to-next-to-leading order enhanced by the resummations of ln(x) terms through the NNLL level and of ln(Q^2) terms by the renormalization group, in excellent agreement with the present world average.