The two-pion production in nucleon-nucleon collisions has been studied by exclusive and kinematically complete experiments from threshold up to $T_p$ = 1.36 GeV at CELSIUS-WASA. At near-threshold energies the total and differential distributions for the $pi^+pi^-$ and $pi^0pi^0$ channels are dominated by Roper excitation and its decay into $Nsigma$ and $Deltapi$ channels. At beam energies $T_p >$ 1.1 GeV the $DeltaDelta$ excitation governs the two-pion production process. In the $pi^+pi^+$ channel evidence is found for the excitation of a higher-lying I=3/2 resonance, favorably the $Delta(1600)$. The isovector fusion processes leading to the deuteron and to quasi-stable $^2$He, respectively, %with the production of an isovector pion-pair exhibit no or only a modest ABC-effect, {it i.e.} low-mass enhancement in the $pipi$-invariant mass spectrum, and can be described by conventional $t$-channel $DeltaDelta$ excitation. On the other hand, the isoscalar fusion process to the deuteron %with the production of an isoscalar pion-pair exhibits a dramatic ABC-effect correlated with a narrow resonance-like energy dependence in the total cross section with a width of only 50 MeV and situated at a mass 90 MeV below the $DeltaDelta$ mass.
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