Measurement of the semileptonic b branching fractions and average b mixing parameter in Z decays

Abstract.

The semileptonic branching fractions for primary and cascade b decays ${\mathrm{BR}}(b\rightarrow \ell^-)$, $\mathrm{BR}(b\rightarrow c\rightarrow \ell^+)$ and $\mathrm{BR}(b\rightarrow\bar c\rightarrow \ell^-$ were measured in hadronic Z decays collected by the DELPHI experiment at LEP. The sample was enriched in b decays using the lifetime information and various techniques were used to separate leptons from direct or cascade b decays. By fitting the momentum spectra of di-leptons in opposite jets, the average b mixing parameter $\bar{\chi}$ was also extracted. The following results have been obtained:

\begin{eqnarray*} \mathrm{BR}({b\rightarrow \ell^- \ell^-}) &=& (10.70\pm0.08 (stat)\pm0.21(syst)_{+0.44}^{-0.30}(model))\% \mathrm{BR}({b\rightarrow c\rightarrow \ell^+})&=& (7.98\pm0.22 (stat)\pm0.21(syst)^{+0.14}_{-0.20}(model))\% \mathrm{BR}({b\rightarrow \bar c \rightarrow \ell^-})&=& (1.61\pm0.20 (stat)\pm0.17(syst)^{+0.30}_{-0.44}(model))\% \bar{\chi} &=& 0.127\pm0.013 (stat)\pm0.005(syst)\pm0.004(model) \end{eqnarray*}