Measurement of the charged multiplicity of [ital Z][sup 0][r arrow][ital b[bar b]] events

Using an impact parameter tag to select an enriched sample of [ital Z][sup 0][r arrow][ital b[bar b]] events, we have measured the difference between the average charged multiplicity of [ital Z][sup 0][r arrow][ital b[bar b]] and [ital Z][sup 0][r arrow] hadrons to be [ital [bar n]][sub [ital b]][minus][ital [bar n]][sub had]=2.24[plus minus]0.30 ( stat )[plus minus]0.33 ( syst ) tracks per event. From this, we have derived [ital [bar n]][sub [ital b]][minus][ital [bar n]][sub [ital u][ital d][ital s]]=3.31[plus minus]0.41[plus minus]0.79. Comparing this measurement with those at lower center-of-mass energies, we find no evidence that [ital [bar n]][sub [ital b]][minus][ital [bar n]][sub [ital u][ital d][ital s]] 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 [ital M][sub [ital b]][sup 2].