Branching ratio measurement of $K_S\to \gamma \gamma$ decay using a pure $\text{ks}$ beam in the KLOE detector

We have analyzed 1.62 fb${}^{-1}$ of $e^{+}{e}^{-}$ collisions at a center of mass energy $\sim M_{\varphi }$ collected by the KLOE experiment at DA$\mathrm{\Phi }$NE. This sample corresponds to a production of $\sim$ 1.7 billion of $\text{ks}$ $\text{kl}$ pairs which allowed us to search for the rare $K_S\to \gamma \gamma$ 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 $\ks \to 2 \pi^0$decayscountedinthesamesample,themeasuredvalueofBR($\ks \to \gamma\gamma$)is(2.27$\pm 0.13(stat.) ^{+0.03}_{-0.04} (syst.)) \times 10^{-6}$,inagreementwith$O(P^4)$ Chiral Perturbation Theory predictions.