Using the high-quality beam of storage rings in combination with a pellet target and the hermetic WASA detector covering practically the full solid angle the two-pion production in nucleon-nucleon collisions has been systematically studied by exclusi
ve and kinematically complete measurements -- first at CELSIUS and subsequently at COSY. These measurements resulted in a detailed understanding of the two-pion production mechanism by $t$-channel meson exchange. The investigation of the ABC effect in double-pionic fusion reactions lead the trace to the observation of a narrow dibaryon resonance with $I(J^P) = 0(3^+)$ about 80 MeV below the nominal mass of the conventional $DeltaDelta$ system. New neutron-proton scattering data, taken with polarized beam at COSY, produce a pole in the coupled $^3D_3$ - $^3G_3$ partial waves at ($2380pm10~-~i~40pm5$) MeV establishing thus the first observation of a genuine $s$-channel dibaryon resonance.
Recently it has been shown by exclusive and kinematically complete experiments that the appearance of a narrow resonance structure in double-pionic fusion reactions is strictly correlated with the appearance of the so-called ABC effect, which denotes
a pronounced low-mass enhancement in the $pipi$-invariant mass spectrum. Whereas the resonance structure got its explanation by the $d^*(2380)$ dibaryonic resonance, a satisfactory explanation for the ABC effect is still pending. In this paper we discuss possible explanations of the ABC effect and their consequences for the internal structure of the $d^*$ dibaryon.
Based on measurements the branching ratios for the decay of the recently discovered dibaryon resonance $d^*(2380)$ into two-pion production channels and into the $np$ channel are evaluated. Possibilities for a decay into the isoscalar single-pion cha
nnel are discussed. Finally also the electromagnetic decay of $d^*(2380)$ is considered.
Exclusive measurements of the $pp to pppi^0pi^0$ reaction have been performed at CELSIUS/WASA at energies from threshold up to $T_p$ = 1.3 GeV. Total and differential cross sections have been obtained. Here we concentrate on energies $T_p ge$ 1 GeV,
where the $DeltaDelta$ excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional $t$-channel $DeltaDelta$ excitation. This holds also for the double-pionic fusion to the quasibound $^2$He. The data are compared to model predictions, which are based on both pion and $rho$ exchange. Total and differential cross sections are at variance with these predictions and call for a profound modification of the $rho$-exchange. A phenomenological modification allowing only a small $rho$ exchange contribution leads to a quantitative description of the data.
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.
The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to $T_p$ = 1.36 GeV. Total and differential cross sections have been obtained for the channels $pnpi^+pi^0$, $pppi^+pi^-$, $pppi^0pi
^0$ and also $nnpi^+pi^+$. For intermediate incident energies $T_p >$ 1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of $DeltaDelta$ excitation, the total $pppi^0pi^0$ cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the $nnpi^+pi^+$ cross section is a factor of five larger than predicted. An isospin decompostion of the total cross sections exhibits a s-channel-like energy dependence in the region of the Roper excitation as well as a significant contribution of an isospin 3/2 resonance other than the $Delta(1232)$. As possible candidates the $Delta(1600)$ and the $Delta(1700)$ are discussed.
