Measurement of the Decay<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mi>L</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mi/><mml:mo>→</mml:mo><mml:mi/><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">π</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="italic">γ</mml:mi><mml:mi mathvariant="italic">γ</mml:mi></mml:math>

We report on a new measurement of the decay ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$ by the KTeV experiment at Fermilab. We determine the ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$ branching ratio to be $(1.68\ifmmode\pm\else\textpm\fi{}0.07\ifmmode\pm\else\textpm\fi{}0.08)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. Our data show the first evidence for a low-mass $\ensuremath{\gamma}\ensuremath{\gamma}$ signal as predicted by recent $O({p}^{6})$ chiral perturbation calculations which include vector meson exchange contributions. From our data, we extract a value for the effective vector coupling ${a}_{V}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}0.72\ifmmode\pm\else\textpm\fi{}0.05\ifmmode\pm\else\textpm\fi{}0.06$.