Upper limit on the η → π + π − branching fraction with the KLOE experiment

bstract Based on an integrated luminosity of 1.61 fb − 1 e + e − collision data collected with the KLOE detector at DAΦNE, the Frascati ϕ -factory, a search for the P - and CP -violating decay η → π + π − has been performed. Radiative ϕ → ηγ decay is exploited to access the η mesons. No signal is observed in the π + π − invariant mass spectrum, and the upper limit on the branching fraction at 90% confidence level is determined to be ℬ( η → π + π − ) < 4 . 9 × 10 − 6 , which is approximately three times smaller than the previous KLOE result. From the combination of these two measurements we get ℬ( η → π + π − ) < 4 . 4 × 10 − 6 at 90% confidence level.