Final results from the KTeV experiment on the decay $K_L\to {\pi }^0\gamma \gamma$

We report on a new measurement of the branching ratio $B(K_L\to {\pi }^0\gamma \gamma )$ using the KTeV detector. We reconstruct 1982 events with an estimated background of 608, that results in $B(K_L\to {\pi }^0\gamma \gamma )=(1.29\pm {0.03}_{\mathrm{stat}}\pm {0.05}_{\mathrm{syst}})\times {10}^{-6}$. We also measure the parameter, $a_V$, which characterizes the strength of vector meson exchange terms in this decay. We find $a_V=-0.31\pm {0.05}_{\mathrm{stat}}\pm {0.07}_{\mathrm{syst}}$. These results utilize the full KTeV data set collected from 1997 to 2000 and supersede earlier KTeV measurements of the branching ratio and $a_V$.

Figure 1

Schematic of the KTeV detector.

Figure 2

Transverse view of the KTeV CsI calorimeter. The smaller blocks are located in the central region with the larger blocks located in the outer region.

Figure 3

The photon shape ${\chi }^2$ variable. (a) The photon shape ${\chi }^2$ variable for $K_L\to {\pi }^0{\pi }^0$ with the dots representing the data and the solid histogram the Monte Carlo simulation. The Monte Carlo simulation was generated using our GEANT-based shower library. (b) The photon shape ${\chi }^2$ variable for $K_L\to {\pi }^0{\pi }^0$ events. The dots are the data and the histogram represents the Monte Carlo simulation using our data-based shower library. The data-based Monte Carlo simulation shows marked improvement over the GEANT-based shower library.

Figure 4

The reconstructed decay vertex position from four photons for all events (histogram) prior to imposing the decay vertex cut. All cuts except for the decay vertex and photon shape ${\chi }^2$ have been applied. The shaded histogram indicates $K_L\to {\pi }^0{\pi }^0$ and $K_L\to {\pi }^0\gamma \gamma$ candidates. The rise at large values is due to $K_L\to {\pi }^0{\pi }^0{\pi }^0$ decays in which one or more photons misses the CsI calorimeter. The position of the cut is indicated by the two vertical lines.

Figure 5

The photon shape ${\chi }^2$ variable. The solid histogram shows the $K_L\to {\pi }^0{\pi }^0{\pi }^0$ Monte Carlo while the dotted histogram is the data. The $K_L\to {\pi }^0\gamma \gamma$ signal Monte Carlo simulation is the shaded histogram. The sum of the signal plus background Monte Carlo simulation agrees well with the data.

Figure 6

The $m_{34}$ mass distribution for after all selection cuts. The dots show the data, the histogram the sum of the signal and background Monte Carlo samples, and the shaded histogram the scaled background Monte Carlo simulation.

Figure 7

The (a) energy and (b) decay vertex for $K_L\to {\pi }^0{\pi }^0$ decays. The dots are the data and the histogram is the Monte Carlo simulation.

Figure 8

The $Y_{\mathrm{Dalitz}}$ distribution for after all selection cuts. The dots show the data, the histogram the sum of the signal and background Monte Carlo simulation, and the shaded histogram the scaled background Monte Carlo simulation.