Observation of doubly-charmed B decays at LEP

Abstract

A search for doubly-charmed B decays with both charmed mesons reconstructed is performed, using about 3.8 million hadronic Z decays recorded with the ALEPH detector at LEP. A clear signal is observed in the channels ${\rm B\rightarrow D_s \bar D}(X)$ and ${\rm B\rightarrow D \bar D}(X)$ (where D can be either a D $^0$ , a D $^+$ or a D $^{*+}$ ), providing the first direct evidence for doubly-charmed B decays involving no ${\rm D_s}$ production. Evidence for associated ${\rm K^0_S}$ and ${\rm K^{\pm}}$ production in the decays ${\rm B\rightarrow D \bar D}(X)$ is also presented and some candidates for completely reconstructed decays ${\rm B\rightarrow D_s \bar D}(n\pi)$ , ${\rm B\rightarrow D \bar D K^0_S}$ and ${\rm B\rightarrow D \bar D K^\pm}$ are observed. Furthermore, candidates for the two-body Cabibbo suppressed decays ${\rm B^0\rightarrow D^{*-}D^{*+}}$ and ${\rm B^-\rightarrow D^{(*)0}D^{(*)-}}$ are also observed. Measurements of the corresponding branching fractions are extracted.