Observation of the rare η→e+e−e+e− decay with the KLOE experiment

We report the first observation of the rare η→e+e−e+e−(γ) decay based on 1.7fb−1 collected by the KLOE experiment at the DAΦNE ϕ-factory. The selection of the e+e−e+e− final state is fully inclusive of radiation. We have identified 362±29 events resulting in a branching ratio of (2.4±0.2stat+bckg±0.1syst)×10−5.