The reaction pd->3He eta at threshold was used to provide a clean source of eta mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay eta->pi+pi-e+e- is measured to be (4.3+/-1.3+/-0.4)x10^-4.
@article{arxiv.hep-ex/0609007,
title = {Measurement of the eta->pi+pi-e+e- decay branching ratio},
author = {WASA Collaboration and Chr. Bargholtz and M. Bashkanov and D. Bogoslawsky and H. Calén and F. Capellaro and H. Clement and L. Demirörs and C. Ekström and K. Fransson and L. Gerén and L. Gustafsson and B. Höistad and G. Ivanov and M. Jacewicz and E. Jiganov and T. Johansson and S. Keleta and I. Koch and S. Kullander and A. Kupsc and A. Kuznetsov and I. V. Laukhin and K. Lindberg and P. Marciniewski and R. Meier and B. Morosov and W. Oelert and C. Pauly and H. Pettersson and Y. Petukhov and A. Povtorejko and R. J. M. Y. Ruber and K. Schönning and W. Scobel and R. Shafigullin and B. Shwartz and T. Skorodko and V. Sopov and J. Stepaniak and V. Tchernyshev and P. -E. Tegnér and P. Thörngren Engblom and V. Tikhomirov and A. Turowiecki and G. J. Wagner and M. Wolke and A. Yamamoto and J. Zabierowski and I. Zartova and J. Złomańczuk},
journal= {arXiv preprint arXiv:hep-ex/0609007},
year = {2012}
}