Studies of Hadronic Event Structure in e+e- Annihilation from 30 GeV to 209 GeV with the L3 Detector

TL;DR

This paper analyzes hadronic event structure in e+e- annihilation using the L3 detector across LEP energies from 30 GeV to 209 GeV to test QCD. It combines detailed measurements of event-shape distributions, jet fractions, and charged-particle spectra with comparisons to Monte Carlo models and analytic QCD predictions, including both perturbative calculations and non-perturbative power corrections. The study provides a precise determination of the strong coupling αs via event shapes, observes the energy evolution consistent with QCD running, and presents evidence for soft-gluon coherence in multiplicity and ξ spectra, while highlighting limitations of power corrections and hadronization models. Overall, the results validate the energy dependence predicted by QCD, demonstrate the importance of coherence effects in jet formation, and emphasize the need for improved theoretical treatments of non-perturbative contributions to sharpen αs extractions.

Abstract

In this Report, QCD results obtained from a study of hadronic event structure in high energy e^+e^- interactions with the L3 detector are presented. The operation of the LEP collider at many different collision energies from 91 GeV to 209 GeV offers a unique opportunity to test QCD by measuring the energy dependence of different observables. The main results concern the measurement of the strong coupling constant, α_s, from hadronic event shapes and the study of effects of soft gluon coherence through charged particle multiplicity and momentum distributions.

Figures (38)

• Figure 1: The eleven regions of L3 detectors as used in the energy measurement for the lep2 configuration. A twelfth region, 5, was present only in earlier set-ups.
• Figure 2: Distributions of scaled visible energy for clusters with linear and non-linear $G$-factors in data at (a) $\sqrt$√s$= 91.2\,\mathrm{\ Ge V} \textrm{Ge V}$ and (b) $\sqrt$√s$= 188.6\,\mathrm{\ Ge V} \textrm{Ge V}$. The points correspond to the measurements and the smooth curves are from fits of a sum of Gaussian distributions as described in the text.
• Figure 3: Jet angular resolutions obtained from the differences of (a,b) polar ($\Delta\Theta=\left|\Theta_2-\Theta_1\right|-\pi$π$$) and (c,d) azimuthal ($\Delta\Phi=\left|\Phi_2-\Phi_1\right|-\pi$π$$) angles of the two jets in two-jet events at (a,c) $\sqrt$√s$=91.2\,\mathrm{\ Ge V} \textrm{Ge V}$ and (b,d) $\sqrt$√s$=188.6\,\mathrm{\ Ge V} \textrm{Ge V}$ with non-linear $G$-factors.
• Figure 4: Distributions of (a) visible energy and (b) number of calorimetric clusters at $\sqrt$√s$=188.6\,\mathrm{\ Ge V} \textrm{Ge V}$. The arrows indicate the selection cuts.
• Figure 5: (a) Distribution of the energy of the most energetic photon candidate at $\sqrt$√s$=200.2\,\mathrm{\ Ge V} \textrm{Ge V}$. The arrow indicates the selection cut. (b) Plot of visible energy vs. energy imbalance along the beam direction for $\sqrt$√s$=200.2\,\mathrm{\ Ge V} \textrm{Ge V}$. the cut used to remove radiative events is indicated.