2018 Impact factor 4.843
Particles and Fields
Eur. Phys. J. C 8, 217-239
DOI 10.1007/s100529901087

A measurement of $R_{\rm b}$ using a double tagging method

The OPAL Collaboration
G.Abbiendi 2 - et al.

2Dipartimento di Fisica dell' Università di Bologna and INFN, I-40126 Bologna, Italy

Received: 28 August 1998 / Published online: 7 April 1999

Abstract
The fraction of ${\rm Z^0 \to b\overline b}$ events in hadronic $\rm Z^0$ decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The ${\rm Z^0 \to b\overline b}$decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of

\begin{displaymath}
R_{\rm b} \equiv \frac{\sigma (\rm e^+e^- \rightarrow\ {\rm
...
 ...rm e^+e^- \rightarrow\ hadrons)}
= 0.2178 \pm 0.0011 \pm 0.0013\end{displaymath}

was obtained, where the first error is statistical and the second systematic. The uncertainty on $R_{\rm c}$, the fraction of ${\rm Z^0
\to c \overline c}$ events in hadronic $\rm Z^0$ decays, is not included in the errors. The dependence on $R_{\rm c}$ is

\begin{displaymath}
\frac{\Delta R_{\rm b}}{R_{\rm b}} = -0.056 \frac{\Delta R_{\rm
c}}{R_{\rm c}} ,\end{displaymath}

where $\Delta R_{\rm c}$ is the deviation of $R_{\rm c}$ from the value 0.172 predicted by the Standard Model. The result for $R_{\rm b}$ agrees with the value of $0.2155 \pm 0.0003$ predicted by the Standard Model.


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