A Measurement of R_b using a Double Tagging Method
The fraction of Z to bbbar events in hadronic Z decays has been measured by
the OPAL experiment using the data collected at LEP between 1992 and 1995.
The Z to bbbar 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 Rb = 0.2178 +- 0.0011 +- 0.0013 was obtained, where the
first error is statistical and the second systematic. The uncertainty on Rc,
the fraction of Z to ccbar events in hadronic Z decays, is not included in
the errors. The dependence on Rc is Delta(Rb)/Rb = -0.056*Delta(Rc)/Rc where
Delta(Rc) is the deviation of Rc from the value 0.172 predicted by the
Standard Model. The result for Rb agrees with the value of 0.2155 +- 0.0003
predicted by the Standard Model.
Parallel session :5
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Plenary session :6a
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