We report on a search for B 0 s ! þ À and B 0 ! þ À decays using proton-antiproton collision data at ffiffi ffi s p ¼ 1:96 TeV corresponding to 10 fb À1 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 BðB 0 ! þ À Þ < 4:6 Â 10 À9 at 95% confidence level. We observe an excess of B 0 s candidates. The probability that the background processes alone could produce such an excess or larger is 0.94%. The probability that the combination of background and the expected standard model rate of B 0 s ! þ À could produce such an excess or larger is 6.8%. These data are used to determine a branching fraction BðB

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