Branching ratio measurement of $K_Sto gamma gamma$ decay using a pure $ks$ beam in the KLOE detector

We have analyzed 1.62 fb$^{-1}$ of $e^{+}e^{-}$ collisions at a center of mass energy $sim M_{phi}$ collected by the KLOE experiment at DA$Phi$NE. This sample corresponds to a production of $sim$ 1.7 billion of $ks$ $kl$ pairs which allowed us to search for the rare $K_Sto gammagamma$ decay. $K_S$ are tagged by the $K_L$ interaction in the calorimeter and the signal is searched for by requiring two additional prompt photons. Strong kinematic requirements reduce the initial 0.5$times 10^6$ events to 2300 candidates from which we extract a signal of 600 $pm$ 35 events. By normalizing to the $ks to 2 pi^0$ decays counted in the same sample, the measured value of BR($ks to gammagamma$) is (2.27 $pm 0.13(stat.) ^{+0.03}_{-0.04} (syst.)) times 10^{-6}$, in agreement with $O(P^4)$ Chiral Perturbation Theory predictions.