Measurement of theKe2+Branching Ratio

The ${K_{e2\mathrm{}}}^{+}$ branching ratio has been measured in a spark-chamber and counter experiment at the Princeton-Pennsylvania accelerator. A $K^{+}$ beam which yielded an average of 500 stopped, identified $K^{+}$ mesons per second was used. Particles which left the stopping region approximately at right angles to the incident beam passed through a series of six thin-plate chambers situated in a 7.5-kG magnetic field which permitted measurement of the particle momentum with a standard deviation of 1.9%. Positrons were identified by a threshold gas Čerenkov counter placed behind the magnetic field. A thick-plate spark chamber was placed behind the Čerenkov counter to permit observation of a particle emerging from that counter and measurement of the particle's total range. The time interval between the $K^{+}$ stop and decay was recorded for each event. Discrimination against background events, arising principally from the ${K_{{\mu }_2}}^{+}$ decay mode, was provided by selection criteria on momentum, range, the time between the $K^{+}$ stop and decay, and by a requirement that the measured position of the track at the entrance to the thick-plate chamber match the extrapolated momentum chamber track. After applying these criteria, we obtain a total of seven events in the momentum interval 242-252 MeV/c in which positrons from ${K_{e2\mathrm{}}}^{+}$ should fall, with 2.8 of these events estimated to be background. This gives a ${K_{e2\mathrm{}}}^{+}$ branching ratio of (${2.1}_{-1.3\mathrm{}}^{+1.8\mathrm{}}$)×${10}^{-5\mathrm{}}$, in agreement with the prediction of $V-A$ theory, which, including a radiative correction, is 1.44×${10}^{-5\mathrm{}}$.