First Observation of the Decay $B_s^0\to D_s^{-}{D}_s^{+}$ and Measurement of Its Branching Ratio

We report the observation of the exclusive decay $B_s^0\to D_s^{-}{D}_s^{+}$ at the 7.5 standard deviation level using $355{\mathrm{pb}}^{-1}$ of data collected by the CDF II detector in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ at the Fermilab Tevatron. We measure the relative branching ratio $\mathcal{B}(B_s^0\to D_s^{-}{D}_s^{+})/\mathcal{B}(B^0\to D^{-}{D}_s^{+})={1.44}_{-0.44}^{+0.48}$. Using the world average value for $\mathcal{B}(B^0\to D^{-}{D}_s^{+})$, we find $\mathcal{B}(B_s^0\to D_s^{-}{D}_s^{+})=({9.4}_{-4.2}^{+4.4})\times {10}^{-3}$. This provides a lower bound $\Delta {\Gamma }_s^{CP}/{\Gamma }_s\ge 2\mathcal{B}(B_s^0\to D_s^{-}{D}_s^{+})>1.2\times {10}^{-2}$ at 95% C.L.

Figure 1

Mass spectra for $B^0\to D^{-}{D}_s^{+}$ (left) and $B_s^0\to D_s^{-}{D}_s^{+}$ (right) where $D_s^{+}\to \varphi {\pi }^{+}$ (top), $D_s^{+}\to {\overline{K}}^{*0}{K}^{+}$ (middle), $D_s^{+}\to {\pi }^{+}{\pi }^{+}{\pi }^{-}$ (bottom). The decomposition of the background into combinatorial background and several backgrounds from $B$ hadron decays is shown. For $B^0$ decays data are represented with dots and error bars, while for the small event yield $B_s^0$ channels a histogram is shown.