Search for the decay K_S->e+e-

TL;DR

This paper reports a search for the rare decay K_S->e+e- using the KLOE detector, setting a new upper limit on its branching ratio that significantly improves previous results, with implications for testing the Standard Model.

Contribution

The study provides the most stringent upper limit to date on the branching ratio of K_S->e+e-, enhancing constraints on rare kaon decays beyond prior experimental bounds.

Findings

BR(K_S->e+e-) < 9.3 x 10^-9 at 90% CL

Improved the upper limit by a factor of ~15 over previous results

Demonstrated effective background rejection techniques

Abstract

We present results of a direct search for the decay K_S->e+e- with the KLOE detector, obtained with a sample of e+e- -> phi -> K_S K_L events produced at DAFNE, the Frascati phi-factory, for an integrated luminosity of 1.9 fb-1. The Standard Model prediction for this decay is BR(K_S->e+e-) = 2 x 10^-14. The search has been performed by tagging the K_S decays with simultaneous detection of a K_L interaction in the calorimeter. Background rejection has been optimized by using both kinematic cuts and particle identification. At the end of the analysis chain we find BR(K_S->e+e-) < 9.3 x 10^-9 at 90% CL, which improves by a factor of ~15 on the previous best result, obtained by CPLEAR experiment.