Measurement of $R_\mathrm{b}$ and $\mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X})$ at LEP using double-tag methods

Abstract.

We present a combined measurement of $R_\mathrm{b} = \Gamma(\mathrm{Z \rightarrow b\bar{b}}) / \Gamma(\mathrm{Z}\rightarrow \mbox{hadrons})$ and the semileptonic branching ratio of b quarks in Z decays, $\mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X})$, using double-tag methods. Two analyses are performed on one million hadronic Z decays collected in 1994 and 1995. The first analysis exploits the capabilities of the silicon microvertex detector. The tagging of b-events is based on the large impact parameter of tracks from weakly decaying b-hadrons with respect to the $\mathrm{e^+ e^-}$ collision point. In the second analysis, a high-$p_t$ lepton tag is used to enhance the b-component in the sample and its momentum spectrum is used to constrain the model dependent uncertainties in the semileptonic b-decay. The analyses are combined in order to provide precise determinations of $R_\mathrm{b}$ and $\mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X})$: \[ R_\mathrm{b} = 0.2174 \pm 0.0015\mbox{ (stat.) } \pm 0.0028\mbox{ (sys.) }; \] \[ \mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X}) = (10.16 \pm 0.13\mbox{ (stat.) } \pm 0.30\mbox{ (sys.) })\%. \]