Measurement of the pn → ppπ0π− Reaction in Search of the ABC Resonance

Abstract

With pd collisions at Tp = 1.2 GeV exclusive measurements of the quasi-free pn → ppπ0π− reaction have been carried out. Using the WASA detector setup at COSY total and differential cross sections have been obtained for the energy region √s = 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 π0π−-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, Δ(1600) and ΔΔ 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 Γ = 70 MeV and I(JP) = 0(3+), which was recently observed in the pn → dπ0π0 reaction, leads to a much improved description of the data at low energies.