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

Abstract

Data from NA31 experiment at CERN have been used to measure the decay K S → γγ. From 69 candidate events of the type K 0 → γγ, 52 events can be attributed to K L → γγ and one event is expected from background processes. The ratio of the relative decay widths R = Γ( K S → γγ) Γ( K L → 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 K L → γγ. This translates into a branching ratio of BR(K S → 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(K L → γγ) 5.86 × 10 −4 ) where the last term accounts for the dependence on the measured K L → γγ branching ratio. The corresponding branching ratio for K S → γγ is BR(K S → γγ) = (2.4 ± 0.9) × 10 −6. The measurement is a sensitive test of chiral perturbation theory and in good agreement with theoretical predictions.