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

¹ Physikalisches Institut, Univ. Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany
² Kepler Center for Astro and Particle Physics, Univ. Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen

^a e-mail: clement@pit.physik.uni-tuebingen.de

Abstract

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, ³He and ⁴He 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→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. From this we conclude that this resonance structure obviously is strong enough to survive even in nuclei.