Measurement of the Decay $K_L\to {\mathit{\pi }}^0\mathit{\gamma }\mathit{\gamma }$

We report on a new measurement of the decay $K_L\to {\pi }^0\gamma \gamma$ by the KTeV experiment at Fermilab. We determine the $K_L\to {\pi }^0\gamma \gamma$ branching ratio to be $(1.68\pm 0.07\pm 0.08)\times {10}^{-6\mathrm{}}$. Our data show the first evidence for a low-mass $\gamma \gamma$ signal as predicted by recent $O\left(p^6\right)$ chiral perturbation calculations which include vector meson exchange contributions. From our data, we extract a value for the effective vector coupling $a_V=-0.72\mathrm{}\pm 0.05\pm 0.06$.