Measurement of the KS->gg branching ratio using a pure KS beam with the KLOE detector

We have searched for the decay KS->gg in a sample of 2x10^9 phi->KS KL decays collected at DAPHNE with an integrated luminosity of 1.9 fb^{-1}. KS are tagged by the KL interaction in the calorimeter. Two prompt photons must also be detected. Kinematic constraints reduce the initial 6x10^5 events to 2740 candidates, from which a signal of 711\pm 35 events is extracted. By normalizing to the KS->2pi^0 decays counted in the same sample, we measure BR(KS->gg)= (2.26\pm0.12_{stat}\pm0.06_{syst})x10^{-6}, in agreement with O(p^4) Chiral Perturbation Theory predictions.