Three-body decay of the d* dibaryon

Under certain circumstances, a three-body decay width can be approximated by an integral involving a product of two off-shell two-body decay widths. This "angle-average" approximation is used to calculate the pi NN decay width of the d*(J(pi) = 3(+),T = 0) dibaryon in a simple Delta(2) model for the most important Feynman diagrams describing pion emissions with baryon-baryon recoil and meson retardation. The decay width is found to be about 0.006 (0.07, 0.5) MeV at the d* mass of 2065 (2100, 2150) MeV for input dynamics derived from the full Bonn potential. The smallness of this width is qualitatively understood as the result of the three-body decay being "third forbidden." The concept of l forbiddenness and the threshold behavior of a three-body decay are further studied in connection with the pi NN decay of the dibaryon d' (J(pi) = 0(-),T = 0 or 2) where the idea of "unfavoredness" has to be introduced. The implications of these results are briefly discussed. [S0556-2813(98)06910-6].