Author Description

A precise measurement of the Ks into two photons decay rate is an important 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 branching ratio, 2.1x 10^(-6), with a few percent precision. The last precise determination of this ratio of 2.71x10^(-6), with a total uncertainty below 3%, comes from NA48. The last mentioned result differs from ChPT prediction of about 30%, indicating possible contributions from higher order corrections. From 2 billion phi mesons collected with KLOE at DAFNE, we have measured this ratio with a 5.3% statistical uncertainty and a 2% systematic error. We obtain a BR result which deviates by 3 sigma's from the previous best determination but confirm ChPT prediction completely neglecting possible higher order corrections.