Double Pion Production in the dd to αππ reaction

Abstract: The reaction dd → 4Heππ has been measured exclusively for the first time. The measurements were performed at Td = 712 MeV and Td = 1029 MeV, with the WASA detector assembly at CELSIUS (Uppsala-Sweden). The aim was to investigate a long standing puzzle, the so called ABC effect. This is a peculiar feature of double pion production in light nuclei collisions. In addition to confirming previous experimental observations in this regard, our results also reveal a strong angular dependence of the pions in the overall center of mass system as well as non-isotropic angular distribution of the low mass enhancement in the ππ invariant mass. The results are qualitatively reproduced by a theoretical model, according to which, the ABC effect is described as a result of kinematical enhancement in the independent production of the pion pairs from two parallel and independent NN → dπ reactions.