Branching ratio measurement of $K_S\to \gamma \gamma$ decay using a pure $\ks$ beam in the KLOE detector

TL;DR

This paper reports a precise measurement of the rare decay $K_S o \gamma\gamma$ using a large dataset from the KLOE experiment, confirming theoretical predictions with improved accuracy.

Contribution

First measurement of the $K_S o \gamma\gamma$ branching ratio using a pure $K_S$ beam at KLOE, providing experimental validation of chiral perturbation theory.

Findings

Measured BR($K_S o \\gamma\\gamma$) = (2.27 ± 0.13(stat.) +0.03/-0.04(syst.)) x 10^{-6}

Result consistent with theoretical predictions from chiral perturbation theory

Analyzed 1.62 fb^{-1} of data, identifying 600 signal events.

Abstract

We have analyzed 1.62 fb$^{-1}$ of $e^{+}e^{-}$ collisions at a center of mass energy $\sim M_{\phi}$ collected by the KLOE experiment at DA$\Phi$NE. This sample corresponds to a production of $\sim$ 1.7 billion of $\ks$ $\kl$ pairs which allowed us to search for the rare $K_S\to \gamma\gamma$ decay. $K_S$ are tagged by the $K_L$ interaction in the calorimeter and the signal is searched for by requiring two additional prompt photons. Strong kinematic requirements reduce the initial 0.5$\times 10^6$ events to 2300 candidates from which we extract a signal of 600 $\pm$ 35 events. By normalizing to the $\ks \to 2 \pi^0$ decays counted in the same sample, the measured value of BR($\ks \to \gamma\gamma$) is (2.27 $\pm 0.13(stat.) ^{+0.03}_{-0.04} (syst.)) \times 10^{-6}$, in agreement with $O(P^4)$ Chiral Perturbation Theory predictions.