Measurement of the KS → γγ branching ratio using a pure KS beam with the KLOE detector

We have searched for the decay KS → γγ in a sample of ~ 2 × 109 → KSKL decays collected at DAΦNE 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 6 × 105 events to 2740 candidates, from which a signal of 711 ± 35 events is extracted. By normalizing to the KS → 2π0 decays counted in the same sample, we measure BR(KS → γγ) = (2.26±0.12stat±0.06syst) × 10−6, in agreement with (p4) Chiral Perturbation Theory predictions.