Limit on an exotic three-body decay of orthopositronium T. Mitsui, K. Maki, S. Asai, Y. Ishisaki, R. Fujimoto, N. Muramoto, T. Sato, Y. Ueda, Y. Yamazaki and S. Orito Europhys. Lett., 33 2 (1996) 111-116 Published online: 01 September 2002 DOI: 10.1209/epl/i1996-00307-8