Upper Limit on Flavor-Changing Neutral-Current Decays of thebQuark

Abstract

We have searched for flavor-changing neutral-current weak decays of the $b$ quark and find $R_B(b\to l^{+}{l}^{-}X)<\mathrm{}0.31\%$ at 90% confidence level. This limit excludes models with the $b$ quark in a left-handed isosinglet. We also place limits on the ratio of the $B^{\pm }$ and $B^0$ semileptonic branching fractions and on $B^0{\bar{B}}^0$ mixing.