Production of $\Xi{_c^0}$ and $\Xi{_b}$ in Z decays and lifetime measurement of $\Xi{_b}$

Abstract.

The charmed strange baryon $\Xi{_c^0}$ was searched for in the decay channel $\Xi{_c^0} \rightarrow \Xi^- \pi^ +$, and the beauty strange baryon $\Xi{_b}$ in the inclusive channel $\Xi_b \rightarrow \Xi^{-} \ell^{-} \bar{\nu} X$, using the 3.5 million hadronic Z events collected by the DELPHI experiment in the years 1992-1995. The $\Xi^-$ was reconstructed through the decay $\Xi^- \rightarrow \Lambda \pi^-$, using a constrained fit method for cascade decays. An iterative discriminant analysis was used for the $\Xi{_c^0}$ and $\Xi{_b}$ selection. The production rates were measured to be $f_{\Xi{_c^0}} \times$BR $(\Xi{_c^0} \rightarrow \Xi^- \pi^ + ) = (4.7 \pm 1.4 ({\rm stat.}) \pm 1.1 ({\rm syst.}))\times 10^{-4}$ per hadronic Z decay, and BR $(b \rightarrow \Xi{_b}) \times$BR $(\Xi{_b} \rightarrow \Xi^- \ell^- X) = (3.0 \pm 1.0({\rm stat.}) \pm 0.3({\rm syst.}))\times 10^{-4}$ for each lepton species (electron or muon). The lifetime of the $\Xi{_b}$ baryon was measured to be $\tau_{\Xi{_b}} = 1.45{^{ + 0.55}_{-0.43}} ({\rm stat.}) \pm 0.13 ({\rm syst.})$ ps. A combination with the previous DELPHI lifetime measurement gives $\tau_{\Xi{_b}} = 1.48{^{ + 0.40}_{-0.31}} ({\rm stat.}) \pm 0.12 ({\rm syst.})$ ps.