Roger A. Freedman, W -Y. P. Hwang, Lawrence Wilets, “Hadron bag model of nucleon-antinucleon annihilation”, Phys. Rev. D, 23:5 (1981), 1103
R. M. Bionta, A. S. Carroll, R. M. Edelstein, D. R. Green, E. J. Makuchowski, S. P. Morrissey, J. S. Russ, J. J. Russell, N. Sharfman, R. B. Sutton, “Search for Narrowp¯pStates in the Reactionπ+p→Δf++p¯p”, Phys. Rev. Lett., 44:14 (1980), 909