Study of the η → π⁺π⁻e⁺e⁻ decay at KLOE

The KLOE experiment has concluded its data taking in March 2006, having acquired an integrated luminosity of 2.5 fb^_1 of e+e- collisions at the center of mass energy around the ø(1020) resonance. The complete data set includes a sample of 100 million η's produced through the radiative decay ø → ηγ and tagged by means of the monochromatic recoil photon. This huge amount of data allows to perform the first test of CP violation in η decays through the study of the rare η → π⁺π^-e⁺e channel, measuring the asymmetry between the π⁺π^- and the e⁺e^- decay planes in the η rest frame (_ϕ). Being this process flavor conserving, such an observation will point to unexpected mechanism of CP violation, thus providing an hint of new physics beyond the Standard Model. A study of the η→ π⁺π^-e⁺e^- decay based on a data sample of 1.7 fb^_1 will be presented. Electrons and pions are identified using both the time of flight to the calorimeter and the momentum of each track. Cuts on track momenta and on photon conversion on the beam pipe allow to significantly reduce the background, with a final 20% contamination. The analysis efficiency for the signal is 8%, dominated by geometrical acceptance. The final sample of about 1600 signal events, 100 times larger than today best measurement, is used to measure both the η→ π⁺π^-e⁺e^-(γ) branching fraction and the asymmetry _ϕ.