ABC Effect in Double-Pionic Fusion – a New Resonance?

Abstract

Abstract. The ABC effect, an intriguing low-mass enhancement in the ππ 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 ∆∆ threshold of 2 m ∆ and its width of only 70 MeV is much less than the 2 Γ ∆ expected from the conventional t-channel ∆∆ 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.

Search for Exotic Hadronic Matter: Tetraquarks, Pentaquarks, Dibaryons and Mesic Nuclei

“…Based on the so far collected data, it was possible to establish that in 88% the D 03 dibaryon decays to ∆∆ and only in 12% to proton-neutron [44,54,55]. Regarding its structure, the result of phenomenological analysis shows that it consists of 67% of hexaquark, 31% of S-wave ∆∆ and 2% of D-wave ∆∆ [44,56].…”