Branching ratio measurement of Ks → γγ 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 ∼ MØcollected by the KLOE experiment at DAPdblNE. This sample corresponds to a pro- duction of ∼ 1.7 billion of KS KL pairs which allowed us to search for the rare KS → γ γ decay. KS are tagged by the KL interaction in the calorimeter and the signal is searched for by requiring two additional prompt photons. Strong kine- matic requirements reduce the initial 0.5×106 events to 2300 candidates from which we extract a signal of 600 ± 35 events. By normalizing to the KS → 2π0 de- cays counted in the same sample, the measured value of BR(KS → γ gamma;) is (2.27 ±0.13(stat.)+0.03-0.04(syst.))×10- 6, in agreement with O(P4) Chiral Perturbation Theory predictions. Key words: e+e- collisions, DAPdblNE, KLOE, rare KS decays, ΧPT.