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

TL;DR

This study measures the eta meson decay branching ratio and searches for CP violation using data from the KLOE experiment, providing the first measurement of a CP-odd asymmetry in this decay mode.

Contribution

First measurement of the CP-odd decay plane asymmetry in eta -> pi+pi-e+e- decay and precise determination of its branching ratio.

Findings

Branching ratio measured as (26.8 +/- 0.9_stat +/- 0.7_syst) x 10^-5.

CP-odd asymmetry A_phi found to be (-0.6 +/- 2.5_stat +/- 1.8_syst) x 10^-2.

Results set constraints on CP violation in eta decays.

Abstract

We have studied the eta->pi+pi-e+e-(gamma) decay using about 1.7 fb^-1 collected by the KLOE experiment at the DAFNE 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 pipi-ee decay planes angular asymmetry, A_phi = (-0.6 +/- 2.5_Stat. +/- 1.8_Syst.) x 10^-2.