An Improved upper limit on the decay K^+ -> pi^+ mu^+ e^-

TL;DR

This paper reports an improved upper limit on the branching ratio of the lepton-family-number-violating decay K^+ -> pi^+ mu^+ e^- based on experimental data, refining previous constraints.

Contribution

The study provides a more stringent upper limit on the decay's branching ratio by combining data from multiple experiments, enhancing the sensitivity to rare decay processes.

Findings

Upper limit on branching ratio: 2.1 x 10^-11 (90% C.L.)

Combined upper limit from multiple experiments: 1.3 x 10^-11 (90% C.L.)

No observation of the decay within experimental sensitivity.

Abstract

Based on results of a search for the lepton-family-number-violating decay $K^+ \to \pi^+\mu^+ e^-$ with data collected by experiment E865 at the Alternating Gradient Synchrotron of Brookhaven National Laboratory, we place an upper limit on the branching ratio at $2.1 \times 10^{-11}$ (90% C.L.). Combining the results with earlier E865 data and those of a previous experiment, E777, an upper limit on the branching ratio of $1.3 \times 10^{-11}$ (90% C.L.) is obtained.