Measurement of Gamma Γ(KS→π+ π- (γ))/Γ(KS→π0 π 0)

We have measured the ratio Rsπ= Γ(KS→π+ π- (γ))/Γ(KS→π0 π 0) with the KLOE detector at the DAFNE e+e- collider. This measurement is fully inclusive with respect to the pi+pi-gamma final state. The sample of over 10^6 two-pion decays of tagged Ks mesons allows a statistical error as low as 0.1% to be obtained. The accuracy is limited by systematic uncertainties, which are estimated primarily from data. We find R=2.236 +/- 0.003(stat) +/- 0.015(syst).