Elsevier

Nuclear Physics A

Volume 958, February 2017, Pages 129-146
Nuclear Physics A

Examination of the nature of the ABC effect

https://doi.org/10.1016/j.nuclphysa.2016.12.001Get rights and content

Abstract

Recently it has been shown by exclusive and kinematically complete experiments that the appearance of a narrow resonance structure in double-pionic fusion reactions is strictly correlated with the appearance of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the ππ-invariant mass spectrum. Whereas the resonance structure got its explanation by the d(2380) dibaryonic resonance, a satisfactory explanation for the ABC effect is still pending. In this paper we discuss possible explanations of the ABC effect and their consequences for the internal structure of the d dibaryon. To this end we examine and review a variety of proposed explanations for the ABC effect, add a new hypothesis and confront all of them with the experimental results for the npdπ0π0 and npnpπ0π0 reactions, which are the most challenging ones for this topic.

Introduction

Recently it was shown that there is a resonance pole at (2380±10)i(40±5)MeV in the D33G33 coupled partial waves based on a SAID partial-wave analysis, which included new data from COSY on the analyzing power of np scattering [1], [2], [3], [4]. This finding matches perfectly with the I(JP)=0(3+) resonance structure observed at s2.37GeV with a width of 70 MeV in the total cross section of the double-pionic fusion reactions pndπ0π0 and pndπ+π [5], [6], [7]. The results from the WASA-at-COSY for the pndπ+π reaction have meanwhile found support by preliminary results from HADES [8].

Having revealed the pole in the np scattering amplitudes means that this resonance structure constitutes an s-channel resonance in the system of two baryons. It has been denoted since then by d(2380) following the nomenclature used for nucleon excitations.

Follow-up measurements of the non-fusion reactions pnppπ0π [9] and pnpnπ0π0 [10] with the WASA detector at COSY and npnpπ+π [11] with the HADES detector at GSI showed that also these reactions, which are partially of isoscalar character, show the d(2380) resonance in agreement with expectations based on isospin coupling.

In addition, WASA measurements revealed that d(2380) is also present in the double-pionic fusion reactions to the helium isotopes pdHe3π0π0, pdHe3π+π, ddHe4π0π0 and ddHe4π+π [12], [13], [14], [15]. This means that obviously d(2380) is stable enough to survive also in a nuclear surrounding. This conclusion is in agreement with the appearance of a dilepton enhancement (DLS puzzle) in heavy-ion collisions [16].

Section snippets

ABC effect

In 1960 Abashian, Booth and Crowe [17] found out that the ππ-invariant mass spectrum in the double-pionic fusion reaction pdHe3ππ exhibits a pronounced low-mass enhancement. Subsequent measurements showed that this enhancement also persists in the fusion reactions to d and 4He [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], if the produced pion pair is of isoscalar nature. Since there was no plausible explanation for this effect, it got named after the initials of the authors

Hypotheses for its explanation

From the Dalitz plots of the double-pionic fusion reactions [5], [6], [7], [12], [14] we know that d(2380) decays predominantly via the ΔΔ system in the intermediate state – with the two Δs being in relative s-wave. This is in accordance with meanwhile numerous theoretical work about this resonance [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. It also is in agreement with the measured branchings of the d decay into the diverse NNππ channels [40].

Therefore it seems likely

Conclusions

We have discussed several possible reasons for the ABC effect. Most of these hypotheses can be tuned to reproduce at least qualitatively the measured low-mass enhancements in the Mππ spectra of double-pionic fusion reactions. The non-occurrence of the ABC effect in the non-fusion reaction npnpπ0π0 has been used as a further constraint to filter out viable solutions.

Most natural and straightforward appears the explanation of the ABC effect as a direct consequence of the vertex function in the d

Acknowledgements

We acknowledge valuable discussions with V. Baru, A. Gal, V. Grishina, C. Hanhart, G. Kälbermann, V. Kukulin, E. Oset, G.J. Wagner and C. Wilkin. This work has been supported by DFG (CL 214/3-1) and STFC (ST/L00478X/1).

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