Determination of alphaS from Hadronic Event Shapes in e+e- Annihilation at 192 < sqrt(s) < 208 GeV

TL;DR

The study tests Quantum Chromodynamics by extracting the strong coupling αs from hadronic event shapes in e+e- annihilation at 192–208 GeV using L3 data and resummed O(αs^2) predictions. By correcting for hadronisation and detector effects and combining with earlier measurements, it demonstrates the running of αs as predicted by QCD and yields a precise αs(mZ) value with quantified experimental and theoretical uncertainties. The work updates previous results with improved theoretical predictions for jet observables and provides important cross-checks of background modelling for high-energy analyses. Overall, the findings reinforce QCD's predicted energy dependence of αs and highlight theoretical uncertainties that motivate further higher-order calculations.

Abstract

Results are presented from a study of the structure of high energy hadronic events recorded by the L3 detector at sqrt(s)>192 GeV. The distributions of several event shape variables are compared to resummed O(alphaS^2) QCD calculations. We determine the strong coupling constant at three average centre-of-mass energies: 194.4, 200.2 and 206.2 GeV. These measurements, combined with previous L3 measurements at lower energies, demonstrate the running of alphaS as expected in QCD and yield alphaS(mZ) = 0.1227 +- 0.0012 +- 0.0058, where the first uncertainty is experimental and the second is theoretical.

Figures (5)

• Figure 1: Corrected distributions for the thrust, $T$, at $<\sqrt{s}>$= 194.4, 200.2 and 206.2 Ge V compared with QCD model predictions. The uncertainties shown are statistical only.
• Figure 2: Corrected distributions for the wide jet broadening, $B_{W}$, at $<\sqrt{s}>$= 194.4, 200.2 and 206.2 Ge V compared with QCD model predictions. The uncertainties shown are statistical only.
• Figure 3: Distribution of a) $<1-T>$ and b) $<B_{W}>$ as a function of the centre-of-mass energy, compared to several QCD models. Lower energy data l3qcd91l3qqgl3qcd133l3qcd161-183l3qcd189 are also presented. The error bars include experimental systematic uncertainties.
• Figure 4: Measured distributions of thrust, $T$, scaled heavy jet mass, $\rho$, total, $B_{T}$, and wide, $B_{W}$, jet broadenings, and $C$-parameter in comparison with QCD predictions at $<\sqrt{s}>$=206.2 $\mathrm{\ Ge V} \textrm{Ge V}$. The error bars include experimental systematic uncertainties.
• Figure 5: Values of $\alpha_{\mathrm{s}}$ determined as a function of $\sqrt{s}$: a) from event shape distributions with experimental uncertainties only. The solid and dashed lines are fits with the energy dependence of $\alpha_{\mathrm{s}}$ as expected from QCD and with constant $\alpha_{\mathrm{s}}$, respectively; b) from the measurement of the $\tau$ branching fractions into leptons l3tau, Z lineshape l3lineshape and event shape distributions. The dashed line is a fit to the QCD evolution function to the measurements made from event shape variables. The band width corresponds to the evolved uncertainty on $\alpha_{\mathrm{s}}(m{\mathrm{Z}}$m_Z$)$.