Abstract
Using the NA48 detector at the CERN SPS, 31 KS→π0γγ candidates with an estimated background of 13.7±3.2 events have been observed. This first observation leads to a branching ratio of BR(KS→π0γγ)z>0.2=(4.9±1.6stat±0.9syst)×10−8 in agreement with Chiral Perturbation Theory predictions.
Highlights
Radiative non-leptonic rare kaon decays have proved to be useful tests for low energy hadron dynamics studied in the framework of the Standard Model by the Chiral Perturbation theory
The theoretical prediction for the branching ratio is 3.8 × 10−8 with higher order corrections expected to be small [1] and is quoted in the kinematic region z = m2γγ/m2K > 0.2 which is free from the overwhelming KS → π0π0 background
KS → π0π0 events can pass the cuts on invariant masses, and especially χ2 > 5400, only if far non-Gaussian tails are present in the energy reconstruction from the liquid krypton calorimeter (LKr) calorimeter
Summary
Radiative non-leptonic rare kaon decays have proved to be useful tests for low energy hadron dynamics studied in the framework of the Standard Model by the Chiral Perturbation theory (χP T ). In the decay KS → π0γγ, the photon pair production is described by an amplitude dominated by a pseudo-scalar meson pole. In χP T this pole is dominated by π0 contribution, and the lowest order amplitude is non-vanishing, in contrast to the similar KL → γγ decay. The theoretical prediction for the branching ratio is 3.8 × 10−8 with higher order corrections expected to be small [1] and is quoted in the kinematic region z = m2γγ/m2K > 0.2 which is free from the overwhelming KS → π0π0 background. A measurement of the branching ratio can provide information about the presence of extra non-pole contributions studied e.g. in [2]. The lowest previously published limit on the branching ratio is BR(KS → π0γγ)z>0.2 < 3.3 × 10−7 at 90% confidence level [3]
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