Using an impact parameter tag to select an enriched sample of ${\mathit{Z}}^0$→bb¯ events, we have measured the difference between the average charged multiplicity of ${\mathit{Z}}^0$→bb¯ and ${\mathit{Z}}^0$→ hadrons to be n${\mathrm{¯}}_{\mathit{b}}$-n${\mathrm{¯}}_{\mathrm{had}}$=2.24±0.30 ( stat )±0.33 ( syst ) tracks per event. From this, we have derived n${\mathrm{¯}}_{\mathit{b}}$-n${\mathrm{¯}}_{\mathit{u}\mathit{d}\mathit{s}}$=3.31±0.41±0.79. Comparing this measurement with those at lower center-of-mass energies, we find no evidence that n${\mathrm{¯}}_{\mathit{b}}$-n${\mathrm{¯}}_{\mathit{u}\mathit{d}\mathit{s}}$ 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 ${\mathit{M}}_{\mathit{b}}^2$.

• Received 5 November 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.3145