New Interpretation of the ABC Effect in Two-Pion Production in NN Collisions

We investigate the basic 2π-fusion reaction pn → d + (ππ)₀ in the energy region T _p = 1–1.4GeV within the framework of a model based on a D ₀₃-dibaryon production and its subsequent decay via two interfering routes: through an intermediate σ meson and through an isovector dibaryon D ₁₂. We demonstrate that the constructive interference of the above two mechanisms gives a strong near-threshold enhancement in the ππ invariant-mass spectrum, i.e., the so-called ABC effect. It is shown that the σ-meson emission from the D ₀₃ dibaryon plays a crucial role in reproducing the shape and position of the ABC enhancement and is tightly related to the idea of chiral symmetry restoration in dense and excited hadronic systems, such as the D ₀₃ state.