Measurement of the forward-backward asymmetry in $\mathrm{Z} \to \mathrm{b} \bar{\mathrm{b}}$ and $\mathrm{Z} \to \mathrm c \bar{\mathrm c}$ decays with leptons

Abstract.

The sample of hadronic Z decays collected by the Aleph detector at Lep in the years 1991-1995 is analysed in order to measure the forward-backward asymmetry in ${\mathrm{Z}} \to{\mathrm{b}} \bar{{\mathrm{b}}}$ and ${\mathrm{Z}} \to{\mathrm c} \bar{{\mathrm c}}$ events and the ${\mathrm{B}}^0{\mathrm{-}}\bar{\mathrm{B}}^0$ average mixing parameter $\bar \chi$. Quark charges are tagged by the charges of electrons and muons produced in ${\mathrm{b}}$ and ${\mathrm c}$ semileptonic decays. Multivariate analyses are used to separate the event flavours and ${\mathrm{b}} \to \ell$/${\mathrm{b}} \to{\mathrm c} \to \ell$ processes. The ${\mathrm{b}}$ and ${\mathrm c}$ quark asymmetries are measured simultaneously; the average mixing parameter and the pole asymmetries are determined to be

$\begin{eqnarray*} \bar \chi &=& 0.1196 \pm 0.0049 \;{\mathrm{(stat.)}} \;{}^{\displaystyle{+0.0043}}_{\displaystyle{-0.0050}} \;{\mathrm{(syst.)}}, {\mathrm{A}}^{0,{\mathrm{b}}}_{\mathrm{FB}} &=& 0.0998 \pm 0.0040 \;{\mathrm{(stat.)}} \; \pm 0.0017 \;{\mathrm{(syst.)}}, {\mathrm{A}}^{0,{\mathrm c}}_{\mathrm{FB}} &=& 0.0732 \pm 0.0053 \;{\mathrm{(stat.)}} \; \pm 0.0037 \;{\mathrm{(syst.)}}. \end{eqnarray*}$

These asymmetries, combined with the Aleph measurements of the ${\mathrm{b}}$ asymmetry using inclusive ${\mathrm{b}}$ hadron decays and of the ${\mathrm c}$ asymmetry using reconstructed ${\mathrm{D}}$ mesons, correspond to a value of the effective electroweak mixing angle of ${\mathrm{sin}}^2\theta^{\mathrm{eff}}_{\mathrm{W}} = 0.23188 \pm 0.00046$.