Measurement of the KS->gg branching ratio using a pure KS beam with the KLOE detector

TL;DR

This study measures the rare decay branching ratio of KS->gg using a pure KS beam at DAPHNE, confirming theoretical predictions with high precision and employing a large dataset of phi decays.

Contribution

First measurement of BR(KS->gg) using a pure KS beam with detailed kinematic analysis and normalization to KS->2pi^0 decays.

Findings

BR(KS->gg) = (2.26 ± 0.12_stat ± 0.06_syst) x 10^{-6}

Results agree with Chiral Perturbation Theory predictions

Large dataset of 2x10^9 phi decays used

Abstract

We have searched for the decay KS->gg in a sample of 2x10^9 phi->KS KL decays collected at DAPHNE with an integrated luminosity of 1.9 fb^{-1}. KS are tagged by the KL interaction in the calorimeter. Two prompt photons must also be detected. Kinematic constraints reduce the initial 6x10^5 events to 2740 candidates, from which a signal of 711\pm 35 events is extracted. By normalizing to the KS->2pi^0 decays counted in the same sample, we measure BR(KS->gg)= (2.26\pm0.12_{stat}\pm0.06_{syst})x10^{-6}, in agreement with O(p^4) Chiral Perturbation Theory predictions.