Search for the decay $B^ + \to K^ + \tau^\mp \mu^\pm$

We present a search for the lepton flavor violating decay B+→K+τ∓μ± using 383×10 6 B¯¯¯B events collected by the BABAR experiment. The branching fraction for this decay can be substantially enhanced in new physics models. The kinematics of the tau from the signal B decay are inferred from the K+, μ, and other B in the event, which is fully reconstructed in one of a variety of hadronic decay modes, allowing the signal B candidate to be fully reconstructed. We observe no excess of events over the expected background and set a limit of B(B+→K+τμ)<7.7×10−5 at 90% confidence level, where the branching fraction is for the sum of the K+τ−μ+ and K+τ+μ− final states. We use this result to improve a model-independent bound on the energy scale of flavor-changing new physics.