Observation of the Decay $K_L\to \mu^+\mu^- \gamma \gamma$
  We have observed the decay $K_L\to \mu^+\mu^- \gamma \gamma$ at the KTeV experiment at Fermilab. This decay presents a formidable background to the search for new physics in $K_L\to\pi^0\mu^+\mu^-$. The 1997 data yielded a sample of 4 signal events, with an expected background of 0.155 $\pm$ 0.081 events. The branching ratio is ${\mathcal B}(K_L\to \mu^+\mu^- \gamma \gamma$) $ = (10.4^{+7.5}_{-5.9} {\rm (stat)} \pm 0.7 {\rm (sys)})\times 10^{-9}$ with $m_{\gamma\gamma} \geq 1 {\rm MeV/c}^2$, consistent with a QED calculation which predicts $(9.1\pm 0.8)\times 10^{-9}$. 
A. Alavi-Harati, et al (KTeV collaboration)