Status and recent developments in NN → NNπ

We present a summary of the recent developments in pion production in NN collisions within chiral effective field theory.     

Models of πNN interactions

A NN model inspired by Quantum Chromodynamics is presented. The model gives an accurate fit to the most recent Arndt NN phase shifts up to 1 GeV and can be applied to study intermediate- and high-energy nuclear reactions.Invited talk to the symposium Mesons and Light Nuclei IV, Bechyn, Czechoslovakia, September 5–10, 1988.This work is supported by the U.S. Department of Energy, Nuclear Physics Division, under contract W-31-109-ENG-38.     

New nuclear three-body clusters φNN

Binding energies of three-body systems of the type φ + 2N are estimated. Due to the strong attraction between φ-meson and nucleon, suggested in different approaches, bound states can appear in systems like φ + np (singlet and triplet), φ + nn and φ + pp. This indicates the principal possibility of the formation of new nuclear clusters.     

将Treiman-Yang判据应用于NN相互作用

本文讨论应用Treiman-Yang判据来研究象n+p→p+p+π^-一类反应的可能性。为此,考虑了相应于确定终态的所有可能单π交换费曼图的贡献。这样就有可能从所有的实验事例中选出对应于某一个图的那些事例。这样的方法也使得有可能将Treiman-Yang判据应用到在终态中有全同粒子的那些过程。     

A search for πNN bound systems

The d(π⁺, π⁻)T⁺⁺⁺ and the d(π⁻, π⁺) T⁻ reactions were studied at an incident pion energy of 260 MeV. Outgoing negative pions were detected at 35° and 55° in the first reaction, and the positive pions at 45° in the second. Indications for structures corresponding to a (π⁺pp) system with a mass of 2003.0±1.0 MeV and a (π⁻nn) system with a mass of 200.1±1.5 MeV were observed with marginal statistical significance. Upper limits for the cross sections are obtained.     

ωNN vertex function and the γπππ anomaly

We show that the empirical ωNN form factor (cut-off 1400–C1500 MeV) can be understood as arising from a combination of a quark model form factor (typical cut-off 700–C800 MeV) and an anomalous form factor ～ q ² arising from the 3π-intermediate state. The anomaly contributes to the Dirac form factor F ₁(q ²) with F ₁(0) = 0, (and sizable), and to the Pauli form factor F ₂(q ²) with F ₂(0) ≠ 0. The resulting tensor coupling F2(0) is sensitive to the cut-off of the pion momenta in the two-loop integrals and turns out to be small for values around 1 GeV. The quark model ωNN tensor coupling F ²(0) vanishes for point-like quarks. The anomaly, however, contributes a non-vanishing tensor coupling which can be seen to effectively enhance the vector coupling in NN models which do not include a tensor coupling.     

NN巴黎势和可能的pp共振态

通过求解NN巴黎光学势的复薛定谔方程,研究了NN系统的近阈束缚和共振行为,得到了一个pp系统的13P0共振态.发现其能量和宽度与最近BES用Breit-Wigner(B-W公式分析J/ψ→γpp衰变的实验数据给出的结果相容.讨论了这一结果的含义及进一步研究的建议     

πNN coupling determined beyond the chiral limit

Within the conventional QCD sum rules, we calculate the πNN coupling constant, g _πN, beyond the chiral limit using two-point correlation function with a pion. For this purpose, we consider the Dirac structure, iγ₅, at m _π ² order in the expansion of the correlator in terms of the pion momentum. For a consistent treatment of the sum rule, we include the linear terms in quark mass as they constitute the same chiral order as m _π ². In this sum rule, we obtain g _πN= 13.3 ± 1.2, which is very close to the empirical πNN coupling. This demonstrates that going beyond the chiral limit is crucial in determining the coupling. Received: 8 July 1999 / Revised version: 20 August 1999     

Polarization phenomena in NN ↔ Dπ reactions

On the basis of a relativistic approach, NN ? Dπ reactions are analyzed in detail. The coherent sum of one-nucleon-exchange and pion-rescattering diagrams is calculated. It is shown that polarization observables are highly sensitive to off-mass-shell effects within the deuteron and that some of these observables can change sign upon taking these effects into account. The effect of the deuteron P wave is also investigated. The results obtained by calculating a full set of observables are compared with experimental data on the reaction pp → Dπ ⁺.     

Polarization observables in (γ,NN) reactions

The formalism of ( γ, NN) reactions is given where the incident photon is polarized and the outgoing-nucleon polarization is detected. Sixteen structure functions and fifteen polarization observables are found in the general case, while only eight structure functions and seven polarization observables survive in coplanar kinematics. Numerical examples are presented for the ¹⁶O(γ, np) and ¹⁶O(γ, pp) reactions. The transitions to the ground state of ¹⁴C and ¹⁴N are calculated in a model where realistic short-range and tensor correlations are taken into account for the np pair, while short-range and long-range correlations are included in a consistent way for pp pairs. The effects of the one-body and two-body components of the nuclear current and the role of correlations in cross-sections and polarizations are studied and discussed. Received: 7 June 2001 / Accepted: 26 September 2001     

ΔπN耦合以及πNN耦合的计算

通过相对论夸克模型并结合介子云的贡献,计算了ΔπN耦合常数以及πNN耦合常数.计算结果表明了介子云对Δ(1232)共振态的性质的重要作用.由此可以明显地看出介子云在低能区的效应.     

ΔπN耦合以及πNN耦合的计算

通过相对论夸克模型并结合介子云的贡献 ,计算了ΔπN耦合常数以及πNN耦合常数 .计算结果表明了介子云对Δ( 1 2 3 2 )共振态的性质的重要作用 .由此可以明显地看出介子云在低能区的效应     

夸克模型中的等效介子交换势与NN散射

用单胶子交换正反夸克对产生模型给出的等效单单介子交换势和等效两π介子交换势计算了NN散射S分波的相移,给出了部份核力的中程吸引的特性.     

πNN过程对Gottfried求和规则破坏的贡献

指出在计算πNN过程对核子结构函数的贡献时,将反冲核子视为物理核子更为合理.对Gottfried求和规则破坏、对质子中海夸克分布u和d不对称u(x)–d(x)等方面的贡献给出了详细的计算,与已有实验进行比较.结果表明π介子交换不是导致这一破坏以及影响核子结构函数的效应的惟一来源.     

Three-Body ΛNN → nNN Nonmesonic Weak Decay Process of Λ Hypernuclei

The fundamental motivation to study the non-mesonic weak decay (NMWD) of Λ hypernuclei is that it provides the unique channel for the information of the baryon–baryon weak interaction in SU3 _f group. The relative strength of the two main channels of NMWD whose decay processes are the Λp → np and Λn → nn has been the long standing puzzle during last several decades. The puzzling status has been settled down recently with the Γ _n / Γ _p value converging to ${\sim 0.5}$ . The large experimental values of Γ _n / Γ _p of the previous measurements turned out to be due to the surprisingly large contribution of the competing three-body NMWD, ΛNN → nNN process. This paper discusses about the first measurement of the branching ratio of the three-body NMWD process and the renormalized intra-nuclear cascade method adopted to disentangle the contribution of the three-body process out of those due to final state interaction. And the prospects of the more accurate measurement of the three-body process also are presented.     

πNN and ΔπN Couplings in a Relativistic Quark Model Approach

Based on a relativistic quark model approach,the decay of Δ^ →π^ p,and the coupling constants of gΔπN and gπNN are studied.Our results are compared with data and empirical values.     

Experiments on the search for the NN-decoupled NN\pi-resonance d’

The status on the search for NN-decoupled -resonances is reviewed with regard to recent experimental searches in the pionic double-charge exchange in nuclei, the two-pion production in nucleon-nucleon collisions, the photo pion-production on the deuteron and the electro pion-production in nuclei.Received: 30 September 2002, Published online: 22 October 2003PACS: 14.20.Gk Baryon resonances with S = 0 - 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV) - 25.40.Ve Other reactions above meson production thresholds (energies 400 MeV)     

Signature of the ηNN configurations in coherent πphotoproduction on the deuteron

The photoproduction of a neutral pion on the deuteron is considered in the energy region around the η threshold, where a bump-like structure was observed at very backward pion angles. Different dynamical aspects which may be responsible for this phenomenon are analysed within a theoretical frame which includes intermediate ηNN configurations. The results show, in particular, that a three-body treatment of the ηNN interaction is of special importance.