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.
The possibility to determine the pLambda scattering length from the final-state interaction in the reaction pp->pKLambda is investigated experimentally. From a transversely polarized measurement, the K analyzing power (A_N) which, in principle, allow
s one to extract the spin triplet scattering length is studied. An unexpected energy dependence of the forward/backward symmetric part of A_N is found. The influence of N*-resonances on the pLambda invariant mass spectrum is investigated by exploiting the large acceptance for the process pp->pKLambda->pKppi and is found to be the main source of uncertainty for determining the pLambda scattering length.
We have reanalyzed the $pi ^{pm} p$ scattering data at low energy in the Coulomb-nuclear interference region as measured by the CHAOS group at TRIUMF with the aim to determine the pion-nucleon $sigma$ term. The resulting value $sigma=(44pm 12)$ MeV,
while in agreement with lattice QCD calculations and compatible with other recent analyses, is significantly lower than that from the GWU-TRIUMF analysis of 2002.
The reaction pp -> K^+ + (Lambda p) has been measured at T_p = 1.953 GeV and Theta = 0 deg with a high missing mass resolution in order to study the Lambda p final state interaction. Narrow S = -1 resonances predicted by bag model calculations are no
t visible in the missing mass spectrum. Small structures observed in a previous experiment are not confirmed. Upper limits for the production cross section of a narrow resonance are deduced for missing masses between 2058 and 2105 MeV/c^2.
The reaction pp -> K+ + (Lambda p) was measured at Tp=1.953 GeV and Theta = 0 deg with a high missing mass resolution in order to study the Lambda p final state interaction. The large final state enhancement near the Lambda p threshold can be describ
ed using the standard Jost-function approach. The singlet and triplet scattering lengths and effective ranges are deduced by fitting simultaneously the Lambda p invariant mass spectrum and the total cross section data of the free Lambda p scattering.
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 paper is obsolete
The ABC effect -- an intriguing low-mass enhancement in the $pipi$ invariant mass spectrum -- is known from inclusive measurements of two-pion production in nuclear fusion reactions. First exclusive measurements carried out at CELSIUS-WASA for the fu
sion reactions leading to d or $^3$He reveal this effect to be a $sigma$ channel phenomenon associated with the formation of a $DeltaDelta$ system in the intermediate state and combined with a resonance-like behavior in the total cross section. Together with the observation that the differential distributions do not change in shape over the resonance region the features fulfill the criteria of an isoscalar s-channel resonance in $pn$ and $NNpipi$ systems, if the two emitted nucleons are bound. It obviously is robust enough to survive in nuclei as a dibaryonic resonance configuration. In this context also the phenomenon of $NDelta$ resonances is reexamined.
