Ju n 20 04 Upper Limit on the η → γγγ Branching Ratio with the KLOE Detector The KLOE Collaboration

We have searched for the C-violating decay η → γγγ in a sample of ∼18 million η mesons produced in φ → ηγ decays, collected with the KLOE detector at the Frascati φ-factory DAΦNE. No signal is observed and we obtain the upper limit BR(η → γγγ) ≤ 1.6 × 10 at 90% CL. The decay η→γγγ is forbidden by charge-conjugation invariance, if the weak interaction is ignored. The present limit for the η → 3γ branching ratio, BR(η → 3γ) ≤ 5× 10 at 95% CL, is based on the result of the GAMS2000 experiment at Serpukhov [1], which studied neutral decays of η mesons from the reaction πp → ηn at a beam momentum of 30 GeV/c. We have searched with KLOE for the decay η→γγγ among four-photon events, corresponding to the two step process φ→ηγ, η→γγγ. The KLOE detector [2–5], operates at the Frascati ee collider DAΦNE [6], which runs at a CM energy W equal to the φ-meson mass, W∼1019.5 MeV. Copious η-meson production is available from the decay φ → ηγ, with a branching ratio of 1.3%. The highest φ-production rate that has been obtained to date was ∼240 φ/s,