Measurement of the Charged Multiplicity of $Z \rightarrow b \overline{b}$ Events

TL;DR

This paper measures the difference in charged particle multiplicity between Z boson decays to bottom quarks and to all hadrons, confirming QCD predictions and the energy independence of this difference.

Contribution

It provides the first precise measurement of the charged multiplicity difference in Z decays to bottom quarks, supporting QCD's asymptotic freedom at the Z mass scale.

Findings

Measured ifference in charged multiplicity: 2.24 b1 0.30 (stat.) b1 0.33 (syst.)

Derived ifference between b and uds quarks: 3.31 b1 0.41 b1 0.79

No energy dependence observed in the multiplicity difference

Abstract

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 $M_b^2$.