Measurement of the K(S) -> gamma gamma branching ratio using a pure K(S) beam with the KLOE detector

We have searched for the decay K(S) --> gamma gamma in a sample of similar to 2 x 10(9) phi --> K(S)K(L) decays collected at DA Phi NE with an integrated luminosity of 1.9 fb(-1). K(S) are tagged by the K(L) interaction in the calorimeter. Two prompt photons must also be detected. Kinematic constraints reduce the initial 6 x 10(5) events to 2740 candidates, from which a signal of 711 +/- 35 events is extracted. By normalizing to the K(S) --> 2 pi(0) decays counted in the same sample, we measure BR (K(S) --> gamma gamma) = (2.26 +/- 0.12(stat) +/- 0.06(syst)) x 10(-6), in agreement with O(p(4)) Chiral Perturbation Theory predictions.