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

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 \[ R_{\rm b} \equiv \frac{\sigma (\rm e^+e^- \rightarrow{\rm b\overline b} )} {\sigma (\rm e^+e^- \rightarrow hadrons)} = 0.2178 \pm 0.0011 \pm 0.0013 \] 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 \[ \frac{\Delta R_{\rm b}}{R_{\rm b}} = -0.056 \frac{\Delta R_{\rm c}}{R_{\rm c}} , \] 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.