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

Abstract

Abstract.With pd collisions at T p = 1.2 GeV exclusive measurements of the quasi-free pn → ppπ 0 π − 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 π 0 π − -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, ∆(1600) and ∆∆ 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(J P ) = 0(3 + ), which was recently observed in the pn → dπ 0 π 0 reaction, leads to a much improved description of the data at low energies.

ABC effect as a signal of chiral symmetry restoration in hadronic collisions

“…Hence, by extracting the σ mass and width from the experimentally measured ABC peak, one is able to judge the degree of chiral symmetry restoration in excited and/or dense hadronic systems. With this interpretation, it is easily understood why the low-M ππ enhancement is not seen in the reaction pn → pp + π − π 0 [31]: although the D 03 resonance is produced there as well, but the σ meson is not. So, we partially rehabilitate the initial interpretation of the ABC effect suggested by Abashian, Booth and Crowe [1], even though the σ-meson generation in our model is not related to the ππ final-state interaction.…”