First observation of the decay $K^{+}\to e^{+}{\nu }_e{\mu }^{+}{\mu }^{-}$

Experiment 865 at the Brookhaven AGS has observed the decay $K^{+}\to e^{+}{\nu }_e{\mu }^{+}{\mu }^{-}$. The branching ratio extracted is $(1.72\pm 0.37(\mathrm{stat})\pm 0.17(\mathrm{syst})\pm 0.19(\mathrm{model}))\times {10}^{-8}$ where the third term in the error results from the use of a model to extrapolate into a kinematic region dominated by background.

Figure 1

Graphs for contributions to $K^{+}\to e^{+}\nu {\mu }^{+}{\mu }^{-}$.

Figure 2

Plan view of the experimental apparatus for E865.

Figure 3

Scatter plot of $m_{e^{+}{\mu }^{+}{\mu }^{-}}$ against $m_{e^{+}\nu {\mu }^{+}{\mu }^{-}}$ for candidate $K^{+}\to e^{+}\nu {\mu }^{+}{\mu }^{-}$ events. Valid events lie in the band $m_{e^{+}\nu {\mu }^{+}{\mu }^{-}}\sim m_K$ indicated on the plot.

Figure 4

Simulation results for the $K^{+}\to e^{+}\nu {\mu }^{+}{\mu }^{-}$ (top left-hand plot) and the computed contributions from the three background sources, $K_{e4}$, $K_{3\pi }$ and accidentals.

Figure 5

Projection onto the $m_{e\nu \mu \mu }$ axis of the data of Fig. 3 (points) and of the fitted distribution (unshaded histogram). The three background contributions are shown separately (shaded histograms).

Figure 6

Distribution in $m_{e\nu \mu \mu }$ of events in three bands of $m_{e\mu \mu }$: (a) $0.2825\mathrm{GeV}/c^2<m_{e\mu \mu }<0.355\mathrm{GeV}/c^2$, (b) $0.355\mathrm{GeV}/c^2<m_{e\mu \mu }<0.4275\mathrm{GeV}/c^2$ and (c) $m_{e\mu \mu }>0.4275\mathrm{GeV}/c^2$. The points are the experimental data and the histogram is the fitted spectrum. The shaded areas show the fitted background contribution.