Hidden dibaryons in one- and two-pion production in NN collisions

Processes of one- and two-pion production in $NN$ collisions are considered in connection with excitation of intermediate dibaryon resonances. In particular, relative contributions of the conventional meson-exchange and dibaryon excitation mechanisms in the reaction $pp \to d \pi^+$ are investigated in detail. Inclusion of the intermediate isovector dibaryon resonances is shown to essentially improve the description of experimental data for this reaction, provided the soft meson-baryon form factors consistent with $\pi N$ elastic scattering are used. Manifestation of the intermediate isoscalar and isovector dibaryons in the two-pion production processes is also studied. The role of the isovector dibaryon resonances in the reaction $pp \to pp \pi\pi$ is discussed for the first time. An explanation of the observed strong differences between two-pion production cross sections in $pn$ and $pp$ collisions based in part on the analysis of dibaryon structure is suggested.