Observation of tree-level B decays with $s\bar{s}$ Production from Gluon Radiation

We report on our search for $B^{-}\to D_s^{(\ast )+}{K}^{-}{\pi }^{-}$, $\bar{B^0} \to D_s^{()+} K_S^0 \pi^-$,and$B^- \to D^{()+}s K^- K^-$decaysin383million$\FourS \to B \Bbar$ events collected by the Babar detector at the PEP2 asymmetric-energy $B$-factory. The decays proceed via a tree-level $b\to c$ quark transition in which a gluon radiates into an $s\bar{s}$ pair. Their branching fractions are measured to be ${\cal B}(B^- \to D^+s K^- \pi^-) = (2.02 \pm 0.13{stat} \pm 0.38{syst}) \times 10^{-4},$ ${\cal B}(B^- \to D^{+}s K^- \pi^-)= (1.67 \pm 0.16{stat} \pm 0.35_{syst}) \times 10^{-4},$ ${\cal B}(\bar{B^0} \to D_s^{+} K_S^0 \pi^-)= (0.55 \pm 0.13_{stat} \pm 0.10_{syst}) \times 10^{-4},$and${\cal B}(B^- \to D_s^{+} K^- K^-) = (0.11 \pm 0.04_{stat} \pm 0.02_{syst}) \times 10^{-4}$. Upper limits at the 90% C.L. are set on $\mathcal{B}(\overline{B^0}\to D_s^{\ast +}{K}_S^0{\pi }^{-})<0.55\times {10}^{-4}$ and $\mathcal{B}(B^{-}\to D_s^{\ast +}{K}^{-}{K}^{-})<0.15\times {10}^{-4}$. We present evidence that the invariant mass distributions of $D_s^{(\ast )+}{K}^{-}$ pairs from $B^{-}\to D_s^{(\ast )+}{K}^{-}{\pi }^{-}$ decays are inconsistent with the phase-space model, suggesting the presence of charm resonances lying below the $D^{()+}_s K^-$ threshold.