Search

With pd collisions at T_p = 1.2 GeV exclusive measurements of the quasi-free pn -> pppi^0pi^- reaction have been carried out. Using the WASA detector setup at COSY total and differential cross sections have been obtained for the energy region &radics = 2.35 - 2.48 GeV. Though this includes the region of the ABC effect and its associated resonance structure, no low-mass enhancement (ABC effect) is found in the pi^0pi^--invariant mass spectrum - in agreement with the constraint from Bose statistics demanding the isovector pion pair to be in relative p-wave. Conventional calculations including t-channel processes for Roper, Delta(1600) and DeltaDelta excitations and their decays, which are well-known from the study of the two-pion production in pp collisions, provide a reasonable description of the data at high energies, but fall low at low energies. From this we conclude that a large contribution from a so far unknown isoscalar low-energy process is missed in the calculations. Inclusion of the ABC resonance at m = 2.37 GeV with Gamma = 70 MeV and I(J^P) = 0(3^+), which was recently observed in the pn -> dpi^0pi^0 reaction, leads to a much improved description of the data at low energies.