Abstract
We report the first observation of the rare η→e+e−e+e−(γ) decay based on 1.7 fb−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.
Highlights
The η → e+e−e+e− decay proceeds through two virtual photons intermediate state with internal photon conversion to e+e− pairs
We report the first observation of the rare η → e+e−e+e−(γ ) decay based on 1.7 fb−1 collected by the KLOE experiment at the DA NE φ-factory
We have identified 362 ± 29 events resulting in a branching ratio of (2.4 ± 0.2stat+bckg ± 0.1syst) × 10−5
Summary
The η → e+e−e+e− decay proceeds through two virtual photons intermediate state with internal photon conversion to e+e− pairs. Conversion decays offer the possibility to precisely measure the virtual photon 4-momentum via the invariant mass of the e+e− pair. The lack of hadrons among the decay products makes the matrix element directly sensitive to the η meson transition form factor [1]. The knowledge of the η meson coupling to virtual photons is important for the calculation of the anomalous magnetic moment of the muon, being pseudoscalar exchange the major contribution to the hadronic light-by-light scattering. The width ratio translates into a branching ratio (BR) BR(η → e+e−e+e−) = 2.59 × 10−5 when the world average of the BR(η → γ γ ) measurements [3] is taken as normalization factor. Double lepton–antilepton η decays have been searched by the CMD-2 and the WASA experiments, obtaining the upper limits at 90% C.L., BR(η → e+e−e+e−) < 6.9 × 10−5 [8] and BR(η → e+e−e+e−) < 9.7 × 10−5 [9], respectively
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