Abstract

ABC effect, an intriguing low-mass enhancement in the π π invariant mass spectrum — known since more than 50 years from inclusive measurements of double-pionic fusion reactions — is reexamined. To this end exclusive and kinematically complete high-statistics experiments of the fusion reactions to d, 3 He and 4 He have been carried out with WASA at COSY. These measurements cover the full energy region, where the ABC effect has been observed previously. They also complement the systematic measurements of nucleon-nucleon induced two-pion production. An isospin decomposition of all three basic double-pionic fusion reactions leading to the deuteron uniquely shows that solely the isoscalar reaction part exhibits the ABC effect tightly correlated with a narrow resonance structure in the total cross section. The peak energy of the resonance structure is about 90 MeV below the nominal ΔΔ threshold of 2 m Δ and its width of only 70 MeV is much less than the 2 Г Δ expected from the conventional t -channel ΔΔ process. Based on angular distributions the quantum numbers I (J P ) = 0(3+ ) have been assigned. In the double-pionic fusion reaction dd →4 Heπ 0 π 0 again the ABC effect is observed to be correlated with the appearance of a resonance-like structure in the total cross section at the same excess energy. From this we conclude that this resonance structure obviously is strong enough to survive even in nuclei.

Highlights

  • The ABC effect, an intriguing low-mass enhancement in the ππ invariant mass spectrum, has first been observed in inclusive measurements of the pd →3He X reaction by Abashian, Booth and Crowe [1]

  • In recent exclusive and kinematically complete measurements of the pn → dπ0π0 reaction it has been demonstrated [12–14] that the ABC effect in this basic double-pionic fusion reaction is correlated with a narrow structure in the total cross section with quantum numbers I(JP) = 0(3+), a mass of 2.37 GeV and a width of about 70 MeV

  • In the double-pionic fusion reaction dd →4Heπ0π0 again the ABC effect is observed to be correlated with the appearance of a resonance-like structure in the total cross section at the same excess energy, with an increased width due to Fermi motion in initial and final nuclei as well as due to collision damping, see Fig. 3 [31, 32]

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Summary

Introduction

The ABC effect, an intriguing low-mass enhancement in the ππ invariant mass spectrum, has first been observed in inclusive measurements of the pd →3He X reaction by Abashian, Booth and Crowe [1]. On the contrary the basic isovector fusion process pp → dπ+π0 exhibits neither an ABC effect nor a narrow resonance structure [14, 15] in this particular energy region — in agreement with the observations in all other two-pion channels of pp initiated reactions [16–27], i.e. of isovector reactions.

Exclusive and Kinematically Complete Experiments
Discussion of Results
Isovector Double-Pionic Fusion
Isoscalar Double-Pionic Fusion
Conclusions and Outlook

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