QCD studies with e+e- annihilation data at 172-189 GeV

TL;DR

This study analyzes hadronic e+e- events at 172–189 GeV with the OPAL detector to test QCD through event shapes, charged multiplicities, and momentum spectra. By fitting O(α_s^2)+NLLA predictions to multiple observables and correcting for hadronization, ISR, and backgrounds, the authors extract α_s at several energies and study its evolution, obtaining α_s(187 GeV)=0.106±0.001(stat)±0.004(syst) and α_s(M_Z)=0.117±0.005. The measured mean charged multiplicity and the ξ_p distribution peak xi_0 are compared to MC models and MLLA predictions, generally aligning with PYTHIA/HERWIG/ARIADNE while showing some deviations in normalization and soft-region yields. Overall, the results corroborate QCD’s applicability at high energies, while highlighting areas where higher-order corrections and hadronization dynamics influence precise shaping of hadronic final states.

Abstract

We have studied hadronic events from e+e- annihilation data at centre-of-mass energies of sqrt{s}=172, 183 and 189 GeV. The total integrated luminosity of the three samples, measured with the OPAL detector, corresponds to 250 pb^-1. We present distributions of event shape variables, charged particle multiplicity and momentum, measured separately in the three data samples. From these we extract measurements of the strong coupling alpha_s, the mean charged particle multiplicity <nch> and the peak position xi_0 in the xi_p=ln(1/x_p) distribution. In general the data are described well by analytic QCD calculations and Monte Carlo models. Our measured values of alpha_s, <nch> and xi_0 are consistent with previous determinations at sqrt{s}=MZ.

Figures (13)

• Figure 1: a) Distribution of reconstructed $\sqrt{s'}$ for the data (full points) with statistical errors. The PYTHIA predictions for all $(\mathrm{Z^0}/\gamma)^{*}$ events (solid line) and for the non-radiative events, $\sqrt{s}-\sqrt{s'_{\mathrm{true}}}<1$ GeV, (dotted line) are also shown. The hatched area indicates the 4-fermion background predicted by the GRC4F Monte Carlo. b) Distribution of the QCD event weight $W_{\mathrm{QCD}}$. The notation is identical to that of a).
• Figure 2: The effect of the $W_{\mathrm{QCD}}$ selection cut on the thrust distribution (see Figure \ref{['fig_1']} for notation). In (a) the thrust distribution, on detector level, is shown before the $W_{\mathrm{QCD}}$ selection is applied. In (b) the same, after the selection on $W_{\mathrm{QCD}}$ is applied.
• Figure 3: Distributions at $\sqrt{s}=189$ GeV of the event shape observables thrust $T$, thrust major $T_{\mathrm{major}}$, thrust minor $T_{\mathrm{minor}}$, aplanarity $A$, C-parameter $C$ and heavy jet mass $M_H$. Experimental statistical errors are delimited by the inner small horizontal bars. The total errors are shown by the outer horizontal error bars. Hadron level predictions from PYTHIA, HERWIG and ARIADNE are also shown.
• Figure 4: Distributions at $\sqrt{s}=189$ GeV of the event shape observables sphericity $S$, oblateness $O$, total jet broadening $B_T$, wide jet broadening $B_W$, and the transition value between 2- and 3-jets $y^D_{23}$. Experimental statistical errors are delimited by the inner small horizontal bars. The total errors are shown by the outer error bars. Hadron level predictions from PYTHIA, HERWIG and ARIADNE are also shown.
• Figure 5: Distribution of the mean thrust $\left< T \right>$ as a function of the centre-of-mass energy, compared to several QCD models.