Observation of the decay mode K_L -> pi^+ pi^- e^+ e^-
  We report on results of an experimental search for the K_L -> pi^+ pi^- e^+ e^- decay mode. We found 13.5 +- 4.0 events and determined its branching ratio to be (4.4 +- 1.3(stat) +- 0.5(syst))*10^{-7}. The result agrees well with the theoretical prediction. 
Y. Takeuchi, Y. Hemmi, H. Kurashige, Y. Matono, K. Murakami, T.   Nomura, H. Sakamoto, N. Sasao, M. Suehiro, Y. Fukushima, Y...