Measurement of the Charged Multiplicity of $Z \rightarrow b   \overline{b}$ Events
  Using an impact parameter tag to select an enriched sample of $Z \rightarrow b \overline{b}$ events, we have measured the difference between the average charged multiplicity of $Z \rightarrow b \overline{b}$ and $Z^0 \rightarrow hadrons$ to be $\overline{n}_{b} - \overline{n}_{had} = 2.24\pm 0.30(\rm{stat.}) \pm 0.33(\rm{syst.})$ tracks per event. {}From this, we have derived $\overline{n}_{b} - \overline{n}_{uds} = 3.31 \pm 0.41 \pm 0.79.$ Comparing this measurement with those at lower center-of-mass energies, we find no evidence that $\overline{n}_{b} - \overline{n}_{uds}$ depends on energy. This result is in agreement with a precise prediction of perturbative QCD, and supports the notion that QCD remains asymptotically free down to the scale $M_b^2$. 
The SLD Collaboration