Measurement of the charged multiplicity of Z0→bb¯ events

Using an impact parameter tag to select an enriched sample of Z0→bb¯ events, we have measured the difference between the average charged multiplicity of Z0→bb¯ and Z0→ hadrons to be n¯b-n¯had=2.24±0.30 ( stat )±0.33 ( syst ) tracks per event. From this, we have derived n¯b-n¯uds=3.31±0.41±0.79. Comparing this measurement with those at lower center-of-mass energies, we find no evidence that n¯b-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 Mb2.