Final results from the KTeV experiment on the decayKL→π0γγ

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 $a_V$.