An improved measurement of $R_b$ using a double tagging method

This paper describes an update of the double tagging measurement of the fraction, R-b, of Z(0) --> events in hadronic Z(0) decays, with statistics improved by including the data collected in 1994. The presence of electrons or muons from semileptonic decays of bottom hadrons and the detection of bottom hadron decay vertices were used together to obtain an event sample enriched in Z(0) --> decays. The efficiency of the event tagging was obtained from the data by comparing the numbers of events having a bottom signature in either one or both thrust hemispheres. Efficiency correlations between opposite event hemispheres are small (< 0.5%) and well understood through comparisons between the real and simulated data samples. A value of R-b = 0.2175 +/- 0.0014 +/- 0.0017 was obtained, where the first error is statistical and the second systematic. The uncertainty on the decay width Gamma(Z(0) --> ) is not included in these errors. The result depends on R-c as follows: Delta R-b/R-b = -0.084 Delta R-c/R-c, where Delta R-c is the deviation of R-c from the value 0.172 predicted by the Standard Model.