Limit on an exotic three-body decay of orthopositroniumT. Mitsui, K. Maki, S. Asai, Y. Ishisaki, R. Fujimoto, N. Muramoto, T. Sato, Y. Ueda, Y. Yamazaki and S. OritoEurophys. Lett., 33 2 (1996) 111-116DOI: https://doi.org/10.1209/epl/i1996-00307-8