Evidence for the decayKL→μ+μ−γγ

Abstract

We have observed the decay KL→μ+μ−γγ at the KTeV experiment at Fermilab. This decay presents a formidable background to the search for new physics in KL→π0μ+μ−. The 1997 data yielded a sample of 4 signal events, with an expected background of 0.155±0.081 events. The branching ratio is B (KL→μ+μ−γγ)=[10.4−5.9+7.5(stat)±0.7(syst)]×10−9 with mγγ>~1 MeV/c2, consistent with a QED calculation which predicts (9.1±0.8)×10−9. Received 23 December 1999DOI:https://doi.org/10.1103/PhysRevD.62.112001©2000 American Physical Society