Measurement of the branching fraction of and search for a CP-violating asymmetry in ?? ? ?+ ?- e+ e- at BESIII

Abstract

Therare decay eta' -> pi(+) pi(-) e(+) e(-) is studied using a sample of1.3 x 10(9) J/Psi events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be (2.42 +/- 0.05(stat) +/- 0.08(syst)) x 10(-3). Due to the inclusion of new data, this result supersedes the last BESIII result on this branching fraction. In addition, the CP-violating asymmetry in the angle between the decay planes of the pi(+)pi(-)-pair and the e(+)e(-)-pair is investigated. A measurable value would indicate physics beyond the standard model; the result is A(CP) = (2.9 +/- 3.7(stat) +/- 1.1(syst))%, which is consistent with the standard model expectation of no CP-violation. The precision is comparable to the asymmetry measurement in the K-L(0) -> pi(+) pi(-) e(+) e(-) decay where the observed (14 +/- 2)% effect is driven by a standard model mechanism.