Ππ Invariant Mass Spectrum Research Articles

Examination of the nature of the ABC effect

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 np→dπ0π0 and np→npπ0π0 reactions, which are the most challenging ones for this topic.

From CELSIUS to COSY: on the observation of a dibaryon resonance

Using a high-quality beam of storage rings in combination with a pellet target and a hermetic WASA detector covering practically the full solid angle, two-pion production in nucleon–nucleon collisions has been systematically studied by exclusive and kinematically complete measurements—first at CELSIUS and subsequently at COSY. These measurements resulted in a detailed understanding of the two-pion production mechanism by t-channel meson exchange. The investigation of the ABC effect, which denotes an unusual low-mass enhancement in the ππ-invariant mass spectrum, in double-pionic fusion reactions led the trace to the observation of a narrow dibaryon resonance with about 80 MeV below the nominal mass of the conventional system. New neutron–proton scattering data, taken with a polarized beam at COSY, produced a pole in the coupled partial waves at () MeV, establishing thus the first observation of a genuine s-channel dibaryon resonance.

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 Tp = 1–1.4GeV within the framework of a model based on a D03-dibaryon production and its subsequent decay via two interfering routes: through an intermediate σ meson and through an isovector dibaryon D12. 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 D03 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 D03 state.

Two-pion production in nucleon–nucleon collisions—Back to dibaryons?

The two-pion production in nucleon–nucleon collisions has been investigated systematically by exclusive and kinematically complete measurements at CELSIUS/WASA and more recently at COSY-TOF and particularly WASA@COSY. The data for all pp-induced isovector production channels in the energy range from threshold up to Tp = 1.4 GeV can be well understood by t-channel Roper, ΔΔ and possibly Δ(1600) excitation. In contrast to that the purely isoscalar reaction channel pn→dπ0π0 exhibits a narrow resonance-like structure with m=2.37GeV and Γ=70MeV in the total cross section, which is correlated with an intriguing low-mass enhancement in the ππ invariant mass spectrum (ABC effect) as well as with the formation of a ΔΔ system in the intermediate state. From the angular distributions the spin and parity of this structure have been derived resulting in I(JP)=0(3+). The consequences of the resonance hypothesis for other reaction channels are discussed. The fact that also the double-pionic fusion reactions to 3He and 4He exhibit a corresponding resonance-like structure in the total cross section correlated with the ABC effect suggests that the underlying conjectured pn resonance is obviously robust enough to survive in nuclei.

WHAT IS THE NATURE OF THE ABC EFFECT?

The ABC effect — an intriguing low-mass enhancement in the ππ invariant mass spectrum — is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA-at-COSY for the fusion reactions leading to d, 3 He and 4 He . They all reveal this effect to be of isoscalar nature and associated with a narrow Lorentzian-shaped structure in the total cross section. The latter could point to the formation of an isoscalar resonance, which couples to the pn channel as well as to an intermediate ΔΔ state.

THE ABC EFFECT AND ITS ENERGY DEPENDENCE IN THE DOUBLE-PIONIC FUSION TO 4He

The double-pionic fusion reactions dd → 4 He π0π0 and dd → 4 He π+π- were measured exclusively and kinematically complete with the WASA detector at COSY. The measurements were performed in the energy range Tp = 0.8 - 1.4 GeV covering thus the full energy region, where the so-called ABC effect – a peculiar low-mass enhancement in the ππ-invariant mass spectrum – was observed previously in inclusive measurements. Our findings for the double-pionic fusion to 4 He are in accordance with the observations for the basic pn → dπ0π0 reaction pointing to the same fundamental mechanism. In both cases we observe that the ABC effect is associated with a peak in the energy dependence of the total cross section. In the 4 He case the peak structure in the total cross section is broadened in comparison to the deuteron case. This is consistent with both Fermi motion of the basic pn system within 4 He and with collision broadening as it is known, e.g., from excitations of the Δ resonance in nuclei.

DOUBLE-PION PRODUCTION IN NUCLEON COLLISIONS ON FEW-BODY SYSTEMS — THE ABC EFFECT AND ITS POSSIBLE ORIGIN

The ABC effect – an intriguing low-mass enhancement in the ππ invariant mass spectrum – is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA@COSY for the fusion reactions leading to d, 3 He and 4 He . They all reveal this effect to be a σ channel phenomenon combined with a resonance-like behavior in the total cross section. The latter points to the formation of an isoscalar dibaryonic resonance, which couples to the pn channel as well as to a ΔΔ intermediate state.

OBSERVATION OF THE ABC EFFECT IN THE FIRST EXCLUSIVE MEASUREMENTS OF pn → dπ0π0

The reaction pn → dπ0π0 constitutes the most fundamental system for the study of the ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of the ππ system in double-pionic fusion to nuclear bound states. The first exclusive measurements of this reaction have now been carried out at CELSIUS-WASA. They exhibit a striking low-mass enhancement in the ππ invariant mass spectrum, prove it to be a σ-channel phenomenon and falsify conventional theoretical models, which predict an associated high-mass enhancement not present in the new exclusive data. In the kinematically complete data sample we observe a ΔΔ excitation, where the interaction between the two Δ obviously plays a crucial role for the generation of the ππ low-mass enhancement.

LOW-MASS ππ ENHANCEMENT IN BARYONIC ππ PRODUCTION: ABC EFFECT REVISED BY EXCLUSIVE MEASUREMENTS

The ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of double-pionic fusion to nuclear bound states - has been investigated since 40 years by inclusive measurements without providing a conclusive interpretation. First exclusive measurements to this topic have now been carried out at CELSIUS-WASA. They confirm a huge ππ threshold enhancement, prove it to be of scalar-isoscalar nature, i.e., a σ-channel phenomenon and reveal the theoretically predicted high-mass enhancement in inclusive spectra to be of πππ rather than of ππ nature - falsifying thus corresponding model predictions for the ABC effect. From the kinematically complete data samples we infer that the interaction between the two Δ particles created in this process is of crucial importance.

Heavy quarkonium [formula omitted] transitions and a possible [formula omitted] state

π + π − transitions of heavy quarkonia, especially the ϒ ( 3 S ) → ϒ ( 1 S ) π + π − decay process, are revisited. In the framework of the Chiral Unitary Theory (ChUT), the S wave ππ final state interaction (FSI) is included. It is found that when an additional intermediate state with J P = 1 + and I = 1 is introduced, not only the ππ invariant mass spectrum and the cos θ π ∗ distribution in the ϒ ( 3 S ) → ϒ ( 1 S ) π + π − process can simultaneously be well explained, but also a consistent description for other bottomonia π + π − transitions can be obtained. As a consequence, the mass and the width of the intermediate state are predicted. From the quark content analysis, this state should be a b b ¯ q q ¯ state.