Measurement of Γ ( K S → π + π − ( γ ) ) / Γ ( K S → π 0 π 0 )

We have measured the ratio R S = Γ(KS → π π(γ))/Γ(KS → π π) with the KLOE detector at the DAΦNE ee collider. This measurement is fully inclusive with respect to the ππγ final state. The sample of over 10 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 S=2.236±0.003stat±0.015syst. The most recent measurement of R S = Γ(KS → π π)/Γ(KS → π π) dates back to 1976. The authors quote an accuracy of 4.3% [1]. This and other results of similar accuracy determine the current world-average value of R S, 2.197±0.026, the fractional error on which is 1.2%. Moreover, the averaging of the existing results is somewhat questionable, since the various experiments have not clearly described their procedures for handling radiative ππγ decays. It is currently of interest to improve the accuracy on R S, with particular attention to an accurate determination of what part of the radiative spectrum is included in the quoted result [2]. The validity of the ∆I = 1/2 rule, empirically established in K, Λ, Σ, and Ξ decays, has never been satisfactorily understood. This lack of understanding is closely connected with the difficulty in relating R(ǫ/ǫ) to the CKM matrix elements [3]. The value of R S also enters into the theoretical calculation for the ratio of branching ratios for the decays KL→μ μ and KL→π π [4]. We describe the first measurement of R S to achieve an accuracy below 1%,