Measurement of the branching ratio and search for a CP violating asymmetry in the η → π+π-e+e-(γ) decay at KLOE

We have studied the η → π+π-e+e-(γ) decay using about 1.7 fb- 1collected by the KLOE experiment at the DAΦNE φsymbol-factory. This corresponds to about 72 millions η mesons produced in φsymbol radiative decays. We have measured the branching ratio, inclusive of radiative effects, with 4% accuracy: BR (η → π+π-e+e-(γ)) = (26.8 ± 0.9Stat .± 0.7Syst .) × 10- 5. We have obtained the first measurement of the CP-odd ππ-ee decay planes angular asymmetry, Aφsymbol= (- 0.6 ± 2.5Stat .± 1.8Syst .) × 10- 2. © 2009 Elsevier B.V. All rights reserved.