Narrow Results

At CELSIUS-WASA the two-pion production in proton-proton collisisons has been measured exclusively from threshold up to the energy regime, where both of the collision partners are expected to be excited to the Δ state. The measurements constitute the first kinematically complete data samples of solid statistics in this energy range. Most of the data have been obtained for the π⁺π^- and π⁰π⁰channels. Whereas at near-threshold energies the differential distributions can be succesfully explained by chiral dynamics and Roper excitation, respectively, the data for the π⁺π^- channel in the ΔΔ region can be described only, if the special configuration (ΔΔ)_0⁺ is assumed. The data for the π⁰π⁰ channel moreover exhibit a low-mass enhancement in the π⁰π⁰ invariant mass spectrum, which is reminiscent of the ABC-effect found in double-pionic fusion to light nuclei.

The reaction pn → dπ⁰π⁰ constitutes the most fundamental system for the study of the ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of the ππ system in double-pionic fusion to nuclear bound states. The first exclusive measurements of this reaction have now been carried out at CELSIUS-WASA. They exhibit a striking low-mass enhancement in the ππ invariant mass spectrum, prove it to be a σ-channel phenomenon and falsify conventional theoretical models, which predict an associated high-mass enhancement not present in the new exclusive data. In the kinematically complete data sample we observe a ΔΔ excitation, where the interaction between the two Δ obviously plays a crucial role for the generation of the ππ low-mass enhancement.

The ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of double-pionic fusion to nuclear bound states - has been investigated since 40 years by inclusive measurements without providing a conclusive interpretation. First exclusive measurements to this topic have now been carried out at CELSIUS-WASA. They confirm a huge ππ threshold enhancement, prove it to be of scalar-isoscalar nature, i.e., a σ-channel phenomenon and reveal the theoretically predicted high-mass enhancement in inclusive spectra to be of πππ rather than of ππ nature - falsifying thus corresponding model predictions for the ABC effect. From the kinematically complete data samples we infer that the interaction between the two Δ particles created in this process is of crucial importance.

The ABC effect – an intriguing low-mass enhancement in the ππ invariant mass spectrum – is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA@COSY for the fusion reactions leading to d, ³He and ⁴He. They all reveal this effect to be a σ channel phenomenon combined with a resonance-like behavior in the total cross section. The latter points to the formation of an isoscalar dibaryonic resonance, which couples to the pn channel as well as to a ΔΔ intermediate state.

The ABC effect, a low-mass enhancement in the invariant ππ mass, is observed in double-pionic fusion reactions leading to a bound nuclear system in the final state. From previous measurements there have been indications that this phenomenon is resctricted to the σ channel of the ππ system. With exclusive measurements of the pp → dπ⁺π⁰ reaction at T_p = 1.1 GeV we demonstrate that, indeed, the ABC effect does not occur in the vector-isovector ππ channel (ρ channel) despite the fact that the ΔΔ excitation is oberved to be the dominant reaction process. We also show that this reaction is well described by a t-channel ΔΔ excitation with the subsequent decay into the vector-isovector ππ channel given by the ρ channel operator.

The ABC effect — an intriguing low-mass enhancement in the ππ invariant mass spectrum — is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA-at-COSY for the fusion reactions leading to d, ³He and ⁴He. They all reveal this effect to be of isoscalar nature and associated with a narrow Lorentzian-shaped structure in the total cross section. The latter could point to the formation of an isoscalar resonance, which couples to the pn channel as well as to an intermediate ΔΔ state.

The double-pionic fusion reactions dd → ⁴Heπ⁰π⁰ and dd → ⁴Heπ⁺π^- were measured exclusively and kinematically complete with the WASA detector at COSY. The measurements were performed in the energy range T_p = 0.8 - 1.4 GeV covering thus the full energy region, where the so-called ABC effect – a peculiar low-mass enhancement in the ππ-invariant mass spectrum – was observed previously in inclusive measurements. Our findings for the double-pionic fusion to ⁴He are in accordance with the observations for the basic pn → dπ⁰π⁰ reaction pointing to the same fundamental mechanism. In both cases we observe that the ABC effect is associated with a peak in the energy dependence of the total cross section. In the ⁴He case the peak structure in the total cross section is broadened in comparison to the deuteron case. This is consistent with both Fermi motion of the basic pn system within ⁴He and with collision broadening as it is known, e.g., from excitations of the Δ resonance in nuclei.

The π⁰π⁰ production in pp-collisions has been investigated in exclusive and kinematically complete measurements from threshold up to T_p = 1.4 GeV. For incident energies T_p > 1 GeV, i.e. in the region beyond the Roper excitation, the ΔΔ excitation process takes over. The data are well explained by the t-channel ΔΔ process dominated by pion exchange. There is no low-mass enhancement (ABC effect) in the π⁰π⁰-invariant mass distribution beyond that given by the conventional t-channel ΔΔ process. This is also true for the limiting case, where the protons are in the quasi-bound ²He state.