Measurement of the Ks into 2 photons ratio using apure Ks beam

A precise measurement of the Ks into two photonsdecay rate is animportant test of Chiral Perturbation Theory predictions.This amplitude has been evaluated at the leading order of ChPT, O(p4), providing the estimate of the corresponding branchingratio, 2.1x 10^(-6), with a few percent precision. The last precise determination of this ratio of 2.71x10^(-6), with a total uncertainty below3%, comes fromNA48. The last mentioned result differs from ChPT prediction of about 30%, indicating possible contributions from higher order corrections.From 2 billion phi mesonscollected with KLOE at DAFNE, we have measured this ratiowith a 5.3% statistical uncertainty and a 2%systematic error. We obtain a BR result which deviates by 3 sigma's from the previousbest determination but confirm ChPT prediction completelyneglecting possible higher order corrections.