Measurement of the forward backward asymmetry of c andb quarks at the Z pole using reconstructed D mesons

Abstract.

A measurement of the forward–backward asymmetry of $e^{+}e^{-} \to c\bar c$ and $e^{+}e^{-} \to b\bar{b}$ on the Z resonance is performed using about 3.5 million hadronic Z decays collected by the DELPHI detector at LEP in the years 1992 to 1995. The heavy quark is tagged by the exclusive reconstruction of several D meson decay modes. The forward–backward asymmetries for c andb quarks at the Z resonance are determined to be: \[ \begin{array}{rcr@{\!\!\;\,}l} A^c_{FB}(\sqrt{s} = 91.235 \,{\rm GeV}) &=& &0.0659\,\, \pm\,\, 0.0094\, (stat)\,\, \pm\,\, 0.0035\, (syst) A^b_{FB} (\sqrt{s} = 91.235 \,{\rm GeV}) &=& &0.0762\,\, \pm\,\, 0.0194\, (stat)\,\, \pm\,\, 0.0085\, (syst) A^c_{FB}(\sqrt{s} = 89.434\,{\rm GeV}) &=&-&0.0496\,\, \pm\,\, 0.0368\, (stat)\,\, \pm\,\, 0.0053\, (syst) A^b_{FB}(\sqrt{s} = 89.434\,{\rm GeV}) &=& &0.0567\,\, \pm\,\, 0.0756\, (stat)\,\, \pm\,\, 0.0117\, (syst) A^c_{FB}(\sqrt{s} = 92.990\,{\rm GeV}) &=& &0.1180\,\, \pm\,\, 0.0318\, (stat)\,\, \pm\,\, 0.0062\, (syst) A^b_{FB}(\sqrt{s} = 92.990\,{\rm GeV}) &=& &0.0882\,\, \pm\,\, 0.0633\, (stat)\,\, \pm\,\, 0.0122\, (syst) \end{array} \] The combination of these results leads to an effective electroweak mixing angle of: \[ \sin^2 \theta ^{lept}_{eff} = 0.2332 \pm 0.0016 \]