A new nonconventional mechanism for the basic 2π-fusion reaction pn → d + (ππ)0 in the energy region Tp = 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(J P ) = 0(3 + ) dibaryon resonance D03 in the pn collision. The interference of two decay channels of this resonance: D03 → d + σ → d + (ππ)0 and D03 → D12 + π → d + (ππ)0 is shown to give a strong near-threshold enhancement in the ππ invariant mass spectrum, which is well known as the ABC effect. The σ-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 σ meson. So, this result might be considered as an indication of partial chiral symmetry restoration in dense and excited quark matter.The famous Abashian-Booth-Crowe (ABC) effect discovered more than 50 years ago [1] is observed in doublepionic fusion reactions [1-3] as a pronounced spectral enhancement of isoscalar nature just above the ππ-production threshold. The effect was initially interpreted [1] as being due to strong ππ rescattering in the scalar-isoscalar channel, associated naturally with the σ meson. However later on the interpretation was left since no narrow resonance with an appropriate mass (m ≃ 300 MeV) was found in ππ scattering at low energies. At the same time, an other interpretation [4] for the ABC effect, based on a generation of two ∆ isobars via the t-channel meson exchange and their subsequent decays with pion emission, was commonly accepted. Although the "t-channel ∆∆" mechanism did not provide quantitative description of the data, it allowed to reproduce the shape of differential cross sections found in the numerous inclusive experiments on double-pionic fusion [4,5].The situation has changed dramatically quite recently, after publication of the results of the first exclusive and kinematically complete experiments for the basic 2π-fusion reaction pn → d + π 0 π 0 done by the CEL-SIUS/WASA [6] and then by the WASA-at-COSY Collaborations [7]. The comparison of the new experimental data with theoretical predictions has demonstrated clearly that the above t-channel ∆∆ model cannot reproduce even the qualitative behaviour of the experimental energy and angular distributions, giving just a low background in the considered energy region (T p = 1.0-1.4 GeV). At the same time, the most intriguing discovery of these exclusive experiments was an observation of a pronounced resonance structure in the total 2π-production cross section. This fact has been interpreted as a generation of the dibaryon resonance D 03 * Electronic address: platonova@nucl-th.sinp.msu.ru † Electronic address: kukulin@nucl-th.sinp.msu.ru in the pn collision, with quantum number...

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Processes of one-and two-pion production in NN collisions are considered in connection with excitation of intermediate dibaryon resonances. In particular, relative contributions of the conventional meson-exchange and dibaryon excitation mechanisms in the reaction pp → dπ + are investigated in detail. Inclusion of the intermediate isovector dibaryon resonances is shown to essentially improve the description of experimental data for this reaction, provided the soft meson-baryon form factors consistent with πN elastic scattering are used. Manifestation of the intermediate isoscalar and isovector dibaryons in the two-pion production processes is also studied. The role of the isovector dibaryon resonances in the reaction pp → ppππ is discussed for the first time. An explanation of the observed strong differences between two-pion production cross sections in pn and pp collisions based in part on the analysis of dibaryon structure is suggested.

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It has been shown for the first time that N N interaction, at least in some partial waves, can be quantitatively described by the superposition of a long-range one-pion exchange and a short-range mechanism based on the complex pole in the N N potential corresponding to the dibaryon resonance in this partial wave. For the partial waves 3 P 2 , 1 D 2 , 3 F 3 and 1 S 0 the parameters of the complex poles that give the best description of the elastic and inelastic phase shifts of N N scattering are very close to the empirical parameters of the corresponding isovector dibaryon resonances detected experimentally.Based on the results obtained, a general conclusion is made about the nature of nuclear force at medium and small internucleon distances.

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