Measurement of R

In 1992 the SLAC Large Detector (SLD) recorded over 10,000 decays of Z^{0} bosons produced in e^{+}e^ {-} collisions at the SLAC Linear Collider (SLC). We search through these decays for Z^ {0}to qoverline{q} events and then tag Z^{0} to boverline{b} events. From these numbers and the estimated tagging efficiencies, we measure R_{b} equiv Gamma(Z ^{0} to boverline{b})/ Gamma(Z^{0} to qoverline {q}). The tagging method used is to count the number of well-measured tracks with highly significant three-dimensional impact parameters. The significance of an impact parameter is defined as the impact parameter divided by the expected uncertainty in that impact parameter. This method should provide an excellent measurement of R _{b} because of SLD's vertex detector, which measures true three-dimensional space points and has a very small inner radius. Also, SLC's interaction point is well defined and stable. Requiring three or more tracks with significance of 3.0 or greater leads to a result of:eqalign{R_{b } &= 0.2270 +/- 0.0075_{rm(stat.) } +/- 0.0091_{rm(syst.)} +/- 0.0040_{(R_ c)}cr Rightarrow R_{b} &= 0.227 +/- 0.012cr}This compares well with the current best measurement, R_{b } = 0.2192 +/- 0.0037, and with the minimal standard model expectation, 0.215 < R_{b} < 0.220, for the allowed range of m _{t} and M_{H }. This measurement is not precise enough to place any significant constraints on the minimal standard model or new physics. As SLD accumulates more statistics and the Monte Carlo simulation is further refined, the uncertainty will decline and we will be able to probe the standard model at a telling level.