Search for $B_s^0\to {\mu }^{+}{\mu }^{-}$ and $B^0\to {\mu }^{+}{\mu }^{-}$ Decays with CDF II

A search has been performed for $B_s^0\to {\mu }^{+}{\mu }^{-}$ and $B^0\to {\mu }^{+}{\mu }^{-}$ decays using $7{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron collider. The observed number of $B^0$ candidates is consistent with background-only expectations and yields an upper limit on the branching fraction of $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<6.0\times {10}^{-9}$ at 95% confidence level. We observe an excess of $B_s^0$ candidates. The probability that the background processes alone could produce such an excess or larger is 0.27%. The probability that the combination of background and the expected standard model rate of $B_s^0\to {\mu }^{+}{\mu }^{-}$ could produce such an excess or larger is 1.9%. These data are used to determine $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})=({1.8}_{-0.9}^{+1.1})\times {10}^{-8}$ and provide an upper limit of $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})<4.0\times {10}^{-8}$ at 95% confidence level.

Figure 1

For the $B_s^0$ and $B^0$ signal regions, the observed number of events (points) is compared to the total expected background (light gray) and its uncertainty (hatched) using the (${\nu }_N,m_{\mu \mu }$) bins from the optimization. The background uncertainty is the quadrature sum of the relevant systematic uncertainties. The top and middle rows show the results in the $B_s^0$ mass signal region for the $CC$ and $CF$ channels, respectively. The bottom row shows the results in the $B^0$ mass signal region for the $CC$ and $CF$ channels combined. The results for the first five ${\nu }_N$ bins are combined (and scaled by 0.2) while the results for the last three bins are each shown separately. Also shown is the expected contribution from $B_s^0\to {\mu }^{+}{\mu }^{-}$ events (dark gray) using a branching fraction that corresponds to the central value from the fit to the data, which is 5.6 times the expected SM value.