Measurement of η meson production in γγ interactions and Γ(η→γγ) with the KLOE detector

"We present a measurement of η meson production in photon-photon interactions produced by electron-positron beams colliding with s√=1 GeV. The measurement is done with the KLOE detector at the ϕ-factory DAΦNE with an integrated luminosity of 0.24 fb−1. The e + e − → e + e − η cross section is measured without detecting the out-going electron and positron, selecting the decays η → π + π − π 0 and η → π 0 π 0 π 0. The most relevant background is due to e + e − → ηγ when the monochromatic photon escapes detection. The cross section for this process is measured as σ(e + e − → ηγ) = (856 ± 8stat ± 16syst) pb. The combined result for the e + e − → e + e − η cross section is σ(e + e − → e + e − η) = (32.72 ± 1.27stat ± 0.70syst) pb. From this we derive the partial width Γ(η → γγ) = (520 ± 20stat ± 13syst) eV. This is in agreement with the world average and is the most precise measurement to date."