Isospin decomposition of the basic double-pionic fusion in the region of the ABC effect

Exclusive and kinematically complete high-statistics measurements of the basic double-pionic fusion reactions $pn\rightarrow d\pi ^{0}\pi ^{0}, pn\rightarrow d\pi ^{+}\pi ^{-}$ and $pp\rightarrow d\pi ^{+}\pi ^{0}$ have been carried out simultaneously over the energy region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the $d\pi ^{0}\pi ^{0}$ channel exhibits the ABC effect, i.e. a low-mass enhancement in the $\pi \pi$ - invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the $d\pi ^{+}\pi ^{-}$ channel shows a smooth behavior consistent with the conventional t -channel $\Delta \Delta$ process. The $d\pi^{+}\pi ^{-}$ data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account.