Observation of Decays $B^0\to D_s^{(*)+}{\pi }^{-}$ and $B^0\to D_s^{(*)-}{K}^{+}$

We report the observation of decays $B^0\to D_s^{(*)+}{\pi }^{-}$ and $B^0\to D_s^{(*)-}{K}^{+}$ in a sample of $230\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ events recorded with the BABAR detector at the SLAC PEP-II asymmetric-energy $e^{+}{e}^{-}$ storage ring. We measure the branching fractions $\mathcal{B}(B^0\to D_s^{+}{\pi }^{-})=\mathbf{(}1.3\pm 0.3(\mathrm{stat})\pm 0.2(\mathrm{syst})\mathbf{)}\times {10}^{-5}$, $\mathcal{B}(B^0\to D_s^{-}{K}^{+})=\mathbf{(}2.5\pm 0.4(\mathrm{stat})\pm 0.4(\mathrm{syst})\mathbf{)}\times {10}^{-5}$, $\mathcal{B}(B^0\to D_s^{*+}{\pi }^{-})=\mathbf{(}2.8\pm 0.6(\mathrm{stat})\pm 0.5(\mathrm{syst})\mathbf{)}\times {10}^{-5}$, and $\mathcal{B}(B^0\to D_s^{*-}{K}^{+})=\mathbf{(}2.0\pm 0.5(\mathrm{stat})\pm 0.4(\mathrm{syst})\mathbf{)}\times {10}^{-5}$. The significances of the measurements to differ from zero are 5, 9, 6, and 5 standard deviations, respectively. This is the first observation of $B^0\to D_s^{+}{\pi }^{-}$, $B^0\to D_s^{*+}{\pi }^{-}$, and $B^0\to D_s^{*-}{K}^{+}$ decays.

Figure 1

Dominant Feynman diagrams for (a) CKM-favored decay $B^0\to D^{(*)-}{\pi }^{+}$, (b) doubly CKM-suppressed decay $B^0\to D^{(*)+}{\pi }^{-}$, and (c) the SU(3) flavor symmetry related decays $B^0\to D_s^{(*)+}{\pi }^{-}$; (d) the color-suppressed $W$-exchange contributions to $B^0\to D^{(*)-}{\pi }^{+}$, (e) $B^0\to D^{(*)+}{\pi }^{-}$, and (f) decay $B^0\to D_s^{(*)-}{K}^{+}$.

Figure 2

(a),(c),(e),(f) $m_{\mathrm{ES}}$ projection of the fit with $|m(D_s^{+})-m(D_s^{+}{)}_{\mathrm{PDG}}|<10\mathrm{MeV}/c^2$ and (b),(d) $m(D_s)$ projection with $5.275<m_{\mathrm{ES}}<5.285\mathrm{GeV}$ for (a),(b) $B^0\to D_s^{+}{\pi }^{-}$, (c),(d) $B^0\to D_s^{-}{K}^{+}$, (e) $B^0\to D_s^{*+}{\pi }^{-}$, and (f) $B^0\to D_s^{*-}{K}^{+}$. Bins with zero events are omitted for clarity. The black solid curve corresponds to the full PDF from the combined fit to all $D_s^{+}$ decay modes. Individual contributions are shown as solid red lines (signal PDF), green dashed lines (combinatorial background), and blue dotted lines (sum of reflection, charmless, and crossfeed backgrounds) curves.