A low-background sample of 10 300 events has been collected for the decay mode $K^{+}\to {\pi }^{+}{e}^{+}{e}^{-}$ by experiment E865 at the Brookhaven Alternating Gradient Synchrotron. The branching ratio is measured to be $[2.94\pm 0.05(\mathrm{stat})\pm 0.13(\mathrm{syst})\pm 0.05(\mathrm{model}\left)\right]\times {10}^{-7\mathrm{}}$, the vector nature of this decay is firmly established, and the decay form factor $f\left(z\right){=f}_0(1+\delta z)\left({z=M}_{\mathrm{ee}}^2{/m}_K^2\right)$ is determined to have $\delta =2.14\mathrm{}\pm 0.13\pm 0.15$. The experiment is described and results discussed in the context of chiral perturbation theory.

• Received 21 July 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.4482