Measurement of the branching ratio and search for a CP violating asymmetry in the eta -> pi(+)pi(-)e(+)e(-)(gamma) decay at KLOE

We have studied the eta -> pi(+)pi(-)e(+)e(-)(gamma) decay using about 1.7 fb(-1) collected by the KLOE experiment at the DA Phi NE phi-factory. This corresponds to about 72 millions eta mesons produced in phi radiative decays. We have measured the branching ratio, inclusive of radiative effects, with 4% accuracy: BR(eta -> pi(+)pi(-)e(+)e(-)(gamma)) = (26.8 +/- 0.9(stat.)+/- 0.7(syst.)) x 10(-5). We have obtained the first measurement of the CP-odd pi pi-ee decay planes angular asymmetry, A(phi) = (-0.6 +/- 2.5(Stat.)+/- 1.8(Syst.)) x 10(-2). (C) 2009 Elsevier B.V. All rights reserved.