A test of chiral perturbation theory from the measurement of the decay KS → γγ

Data from NA31 experiment at CERN have been used to measure the decay KS → γγ. From 69 candidate events of the type K0 → γγ, 52 events can be attributed to KL → γγ and one event is expected from background processes. The ratio of the relative decay widths R = Γ(KS → γγ)Γ(KL → γγ) is measured to be R = 2.2 ± 1.0(stat.) ± 0.3(syst.) ± 0.2(ext.) where the external systematic error is due to the experimental uncertainty in the branching ratio of the decay KL → γγ. This translates into a branching ratio of BR(KS → γγ) = (2.2 ± 1.1) × 10−6. If these data are combined with the published data from an earlier data taking period with the same experiment, the ratio R is determined to be R = 2.35 ± 0.75(stat.) ± 0.4(syst.) + 7.5 · (1 − BR(KL → γγ)5.86 × 10−4) where the last term accounts for the dependence on the measured KL → γγ branching ratio. The corresponding branching ratio for KS → γγ is BR(KS → γγ) = (2.4 ± 0.9) × 10−6. The measurement is a sensitive test of chiral perturbation theory and in good agreement with theoretical predictions.