Measurement of the decay $K_L\to {\pi }^0{e}^{+}{e}^{-}\gamma$

We report on a new measurement of the branching ratio $\mathrm{B}(K_L\to {\pi }^0{e}^{+}{e}^{-}\gamma )$ using the KTeV detector. This analysis uses the full KTeV data set collected from 1997 to 2000. We reconstruct 139 events over a background of 14, which results in $\mathrm{B}(K_L\to {\pi }^0{e}^{+}{e}^{-}\gamma )=(1.62\pm {0.14}_{\mathrm{stat}}\pm {0.09}_{\mathrm{syst}})\times {10}^{-8}$. This result supersedes the earlier KTeV measurement of this branching ratio.

Figure 1

Schematic of the KTeV detector.

Figure 2

The $K_L\to {\pi }^0{\pi }_D^0$ invariant mass distribution for data (dots) and $K_L\to {\pi }^0{\pi }_D^0$ Monte Carlo (solid histogram).

Figure 3

The photon shape variable shape ${\chi }^2$. The top plot shows the shape ${\chi }^2$ variable for $K_L\to {\pi }^0{\pi }_D^0$ with the dots representing the data and the solid histogram the Monte Carlo. In the bottom histogram the dots are the data after removing the $K_L\to {\pi }^0{\pi }_D^0$ events, while the solid histogram shows the $K_L\to {\pi }^0{\pi }^0{\pi }_D^0$ Monte Carlo. The dashed histogram represents the sum of the signal and background Monte Carlo samples.

Figure 4

The pp0kine variable plotted versus the $\gamma \gamma \gamma$ invariant mass for (a) $K_L\to {\pi }^0{\pi }_D^0$ and (b) $K_L\to {\pi }^0{\pi }^0{\pi }_D^0$ Monte Carlo events. The dark line represents the cut indicated in the text.

Figure 5

The minimum distance between the projection of the upstream electron track segment to the CsI calorimeter and any photon candidate. The dots represent the data and the solid histogram the sum of the background and signal Monte Carlo. The enhancement at low values of the minimum distance is the result of bremsstrahlung production.

Figure 6

The $e^{+}{e}^{-}\gamma \gamma \gamma$ invariant mass for events passing all selection criteria. The dots represent the data, while the dashed histogram represents the sum of the signal and background Monte Carlo. The background Monte Carlo is indicated by the shaded histogram.

Figure 7

The Dalitz variables used in the fit for $a_V$: (a) $m_{e^{+}{e}^{-}\gamma }$, (b) $Y$, and (c) $m_{e^{+}{e}^{-}}$. The dots are the data. The dashed histogram is the sum of the background and signal Monte Carlo, and the filled histogram is the background Monte Carlo.