ABC Effect and Resonance Structure in the Double-Pionic Fusion to ${}^3$He

Abstract

Exclusive and kinematically complete measurements of the double pionic fusion to ${}^3$He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the $\ddot{I}€\ddot{I}€$-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observations in the basic $pn\to d\ddot{I}{€}^0\ddot{I}{€}^0$ reaction and in the $dd{\to }^4$He$\ddot{I}{€}^0\ddot{I}{€}^0$ reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of $d^{\ast }$ resonance formation during the reaction process in addition to the conventional $t$-channel $\hat{I}”\hat{I}”$ mechanism. The deduced $d^{\ast }$ resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.