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

Abstract

A new nonconventional mechanism for the basic 2{\pi}-fusion reaction pn --> d+({\pi}{\pi})_0 in the energy region T_p = 1.0-1.4 GeV is suggested. The mechanism is aimed at providing a consistent explanation for the comprehensive experimental studies of this reaction in exclusive setting done recently by the WASA-at-COSY Collaboration. The basic assumption of the model proposed is the production of the I(JP) = 0(3+) dibaryon resonance D03 in the pn collision. The interference of two decay channels of this resonance: D_03 --> d + {\sigma} --> d + ({\pi}{\pi})_0 and D_03 --> D_12 + {\pi} --> d + ({\pi}{\pi})_0 is shown to give a strong near-threshold enhancement in the {\pi}{\pi} invariant mass spectrum, which is well known as the ABC effect. The {\sigma}-meson parameters found to reproduce the ABC enhancement are in a general agreement with models which predict the chiral symmetry restoration at high excitation energy and/or high density of matter, although they are essentially less than those accepted for the free {\sigma} meson. So, this result might be considered as an indication of partial chiral symmetry restoration in dense and excited quark matter.