Search for the decay B+ -> K+ tau-/+ mu+/-

TL;DR

This study searches for a rare lepton flavor violating decay of B+ mesons into K+ tau and mu, finding no evidence and setting a new upper limit that constrains new physics models.

Contribution

The paper provides the first search for B+ --> K+ tau mu decay using a large dataset and sets a new upper limit, improving bounds on flavor-changing new physics.

Findings

No excess events observed over background.

Upper limit on branching fraction: < 7.7 x 10^-5.

Improved bounds on new physics energy scale.

Abstract

We present a search for the lepton flavor violating decay B+ --> K+ tau-/+ mu+/- using 383 million B Bbar 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+, mu, 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+ tau mu) < 7.7 x 10^-5 at 90% confidence level, where the branching fraction is for the sum of the K+ tau- mu+ and K+ tau+ mu- final states. We use this result to improve a model-independent bound on the energy scale of flavor-changing new physics.