ABC Effect and Resonance Structure in the Double-Pionic Fusion to $^3$He
  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 $\pi\pi$-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observations in the basic $pn \to d \pi^0\pi^0$ reaction and in the $dd \to ^4$He$\pi^0\pi^0$ 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 $d^*$ resonance formation during the reaction process in addition to the conventional $t$-channel $\Delta\Delta$ mechanism. The deduced $d^*$ resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei. 
P. Adlarson, W. Augustyniak, W. Bardan, M. Bashkanov, F. S. Bergmann,   M. Ber{\l}owski, H. Bhatt, A. Bondar, M. B\"uscher, ...
nucl-ex hep-ex