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

Abstract

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...

Three Body Calculation of theΔΔDibaryon CandidateD03(2370)

“…For π-(πN N ) D12(2150) , the ∆-dominated πN interaction should reduce the ℓ = 1 upward shift from 300 MeV (no interaction) to about 150 MeV, so that the effective threshold here becomes somewhat lower than the ℓ = 0 ∆∆ threshold. This singles out ∆(1232) and D 12 (2150) as the most likely fermionic degrees of freedom of which pions may benefit in forming the I(J P ) = 0(3 + ) ππN N [17]. We also note that by assigning s-wave N ∆ structure to D 12 , both angularmomentum and isospin recoupling coefficients for transforming πD 12 (2150) with p-wave pion in a I(J P ) = 0(3 + ) state into s-wave ∆∆ are equal to 1, thus maximizing the coupling between these channels.…”

Examination of the nature of the ABC effect

“…In addition, investigation of such a decay branch into the N N π system can resolve the question, whether the main decay route is d * (2380) → ∆∆ or d * (2380) → D 12 π as anticipated in Refs. [31,32,59]. Since D 12 has a sizeable decay branch into N N , the route via D 12 will feed also the N N π channel, whereas such a feeding is very unlikely to happen with sizeable intensity in case of an intermediate isoscalar ∆∆ system.…”

“…Recently Platonova and Kukulin proposed an alternative explanation of the ABC effect [59,60]. They consider two possible decay branches for the d * (2380) resonance.…”

“…7(b)). The strong mass reduction compared to the accepted value [73] has been assigned to chiral symmetry restoration inside the d * dibaryon [59].…”

“…[59] momentum dependent decay widths containing also vertex functions analogous to that in eq. (16) The second decay route constitutes a really new piece in trying to explain the ABC effect.…”