Critical Stability in Nuclei – An Old Problem Revisited

In this paper we will focus on the nucleon-nucleon interaction in relative S-states. The ¹S₀ interaction is known to be close to that for critical two-body binding. We will discuss two approaches to the NN interaction, which are equivalent on-shell but not off-shell. There is a well-defined transformation between these approaches [1]. One (my preferred approach) is to minimize the tensor forces far off-shell, which leads to more rapid convergence, but at the price of significant non-locality. This approach is used in a model of relativistic tensor quenching of OPEP [2]. With non-local NN interactions it is possible to fit not only NN observables, but also the NNN ground-state energies [3]. The other approach is to maximize the role of the and keep the interaction as local as possible [4]. This approach is also internally consistent, but requires additional NNN interactions to fit nuclear data. Also, we discuss briefly a so-called low momentum interaction [5, 6], which fits S-wave phase shifts quite well. This interaction is strongly non-local, and it can be approximated by a schematic separable interaction. Finally, we mention the Nambu–Jona-Lasinio model [7] and a good approximation, the Hulthen potential, which provides some insight into the near critical two-body binding.     

Analysis of the low-energy ηNN-dynamics within a three-body formalism

The interaction of an η-meson with two nucleons is studied within a three-body approach. The major features of the ηNN-system in the low-energy region are accounted for by using a s-wave separable ansatz for the two-body ηN and NN amplitudes. The calculation is confined to the (J ^π;T) = (0^-;1) and (1^-;0) configurations which are assumed to be the most promising candidates for virtual or resonant ηNN-states. The eigenvalue three-body equation is continued analytically into the nonphysical sheets by contour deformation. The position of the poles of the three-body scattering matrix as a function of the ηN-interaction strength is investigated. The corresponding trajectory, starting on the physical sheet, moves around the ηNN three-body threshold and continues away from the physical area giving rise to virtual ηNN-states. The search for poles on the nonphysical sheets adjacent directly to the upper rim of the real energy axis gives a negative result. Thus no low-lying s-wave ηNN-resonances were found. The possible influence of virtual poles on the low-energy ηNN-scattering is discussed. Received: 27 June 2000 / Accepted: 3 August 2000     

Towards a field theoretic understanding of NN → NNπ

We study the production amplitude for the reaction NN → NNπ up to next-to-leading order in chiral perturbation theory using a counting scheme that takes into account the large scale introduced by the initial momentum. In particular, we investigate a subtlety that arises once the leading loop contributions are convoluted with the NN wave functions as demanded by the non-perturbative nature of the NN interaction. We show how to properly identify the irreducible contribution of loop diagrams in such type of reaction. The net effect of the inclusion of all next-to-leading-order loops is to enhance the leading rescattering amplitude by a factor of 4/3, bringing its contribution to the cross-section for pp → dπ⁺ close to the experimental value.     

Resonant behaviour in double charge exchange reaction of πmesons on the nuclear photoemulsion

The invariant-mass spectra of the ppπ^- and pp systems produced in the double charge exchange (DCX) of positively charged pions on photoemulsion are analysed. A pronounced peak is observed in the ppπ^- invariant-mass spectrum, while the M_pp spectrum exhibits a strong Migdal-Watson effect of the proton-proton final-state interaction. These findings are in favor of the NN-decoupled NNπ pseudoscalar resonance with T = 0 called d'.     

Calculating the characteristics of neutron-deuteron and proton-deuteron systems in a two-body potential model

³H and ³He nuclei are considered in a two-body model (³H = n + d; ³He = p + d). Two independent approaches are used: in the first, interaction is described by the folding potential, while NN potentials are taken in the Hulthen form with allowance for violations of isotopic invariance. The second approach features phenomenological Hulthen and Yukawa Nd potentials used as Nd interaction. In both approaches, the binding energies, vertex constants, and asymptotic normalization coefficients in the N+d channel are calculated.     

Electromagnetic Few-Body Response Functions with the Lorentz IntegralTransform Method

 The inclusive reactions and are studied with the Lorentz integral transform method. The method allows the inclusion of the full final-state interaction without explicit knowledge of the continuum states. Features of the inversion procedure are discussed in this work. The technique for the calculation of the matrix elements of the Hamiltonian is described. The total photoabsorption cross section of ³H is calculated with a realistic super-soft-core NN force. In the threshold region the cross section is rather similar to that obtained with central forces only, while in the peak the realistic NN interaction leads to more strength. For the longitudinal (e, e′) response we extend our calculation with a semirealistic force to MeV/c. Rather good agreement with experimental data is found. The accuracy of the quasielastic response approximation is discussed. Received August 15, 1998; revised February 22, 1999; accepted for publication March 9, 1999     

Parameterization of a potential function for the Ca2+ −Ne and Ca2+ −N2 interactions using high-level ab initio data

Ab initio calculations have been carried out at the CCSD(T)/aug-cc-pVTZ level on di-nitrogen and neon interacting with a calcium (II) cation. The interaction of nitrogen with the Ca²⁺ cation is examined for end-on and side-on approaches. The two adjustable parameters in the TrAZ potential (A ^Ca2+-Ne , b ^Ca2+-Ne ) are fitted to the Ca²⁺-Ne potential energy curve and combination rules are then applied in reverse to get A ^Ca2+-Ca2+ , b ^Ca2+-Ca2+ . Combining these with values of A^NN, 6^NN enables the Ca²⁺-N₂ potential energy curves to be predicted a priori. Agreement with the ab initio data is found to improve dramatically when the anisotropy of the nitrogen polarizability is included in the induced interaction energy. Other combination rules are tried but are not found to have an impact on the agreement between the a priori potential and the ab initio data     

Light nuclei in <Emphasis Type="Italic">ab initio</Emphasis> approach with realistic inverse scattering <Emphasis Type="Italic">NN</Emphasis>-interaction

We discuss the studies of light nuclei in ab initio No-core Full Configuration approach based on extrapolations to the infinite model space of large-scale No-core Shell Model calculations on supercomputers. The convergence at the end of p shell and beginning of the sd shell can be achieved if only reasonable soft enough NN interactions are used. In particular, good predictions are obtained with a realistic JISP16 NN interaction obtained in J-matrix inverse scattering approach and fitted to reproduce light nuclei observables without three-nucleon forces. We discuss the current status of this NN interaction and its recent development.     

New short-range electromagnetic current in the deuteron

A new model for short-range isoscalar current in the deuteron and NN system is developed and tested on a variety of isoscalar magnetic observables such as the deuteron magnetic moment, magnetic form factor, and the circular polarization of photons at radiative capture at thermal neutron energies. The model for electromagnetic two-nucleon current proposed in the paper is based on generation of an intermediate dibaryon in the short-range NN interaction. This intermediate dibaryon, in turn, is treated within the new model for intermediate and short-range NN interaction recently proposed by the present authors. The transition current model developed here satisfies the current conservation relation by the construction. Our calculations have demonstrated that the new current model, using only one parameter (with a clear physical meaning), is able to describe, in very good agreement with the experimental data, three basic deuteron observables of magnetic type, viz., the magnetic moment, the photon circular polarization in the process, and the structure function B(Q ²) up to Q ² ≃ 60 fm⁻². The text was submitted by the authors in English.     

πNN Dynamics in pp → pp,pp →πd and πd →πd Processes Below 1 GeV

pp and πd scattering and pp →πd reaction processes at incident proton laboratory energies T _L ≤ 1 GeV are studied in the framework of πNN dynamics. For this purpose three-body calculations are performed with the πN interaction in the P ₁₁, P ₃₃, S ₁₁ and S ₃₁ states and NN forces in the ³ S ₁ -³ D ₁, ¹ S ₀, and ³ P ₂ states. In addition, the backward-going pion contribution at the πNΔ vertex, the πN-ρN coupling, the heavy-meson exchanges in the NN → NN driving term, and the effect of the off-shell structure in the πN-ρN P ₃₃ interaction are taken into account. Received March 28, 1994; revised July 29, 1994; accepted for publication August 31, 1994     

Subthreshold K + meson production in proton–nucleus reactions and nucleon spectral function

The inclusive K ⁺ meson production in proton–nucleus collisions in the near threshold and subthreshold energy regimes is analyzed with respect to the one–step (pN→K ⁺ YN, Y=Λ,Σ) and two–step (pN→NNπ, NN2π; πN→K ⁺ Y) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account the struck target nucleon removal energy and momentum distribution (nucleon spectral function), extracted from recent quasielastic electron scattering experiments and from many–body calculations with realistic models of the NN interaction. Comparison of the model calculations of the K ⁺ total and double differential cross sections for the reaction p+C ¹² with the existing experimental data is given, illustrating both the relative role of the primary and secondary production channels at considered incident energies and those features of the cross sections which are sensitive to the high momentum and high removal energy part of the nucleon spectral function that is governed by nucleon–nucleon short–range and tensor correlations. It is found that the in–medium modifications of the available for pion and kaon production invariant energies squared due to the respective optical potentials are needed to account for considered experimental data. Received: 2 April 1997 / Revised version: 7 August 1997     

Leading-order 2πγ exchange NN interaction: Central potentials proportional to g<Subscript>A</Subscript><Superscript>0</Superscript> and g<Subscript>A</Subscript><Superscript>2</Superscript>

We calculate at two-loop order in chiral perturbation theory the electromagnetic corrections to the leading-order 2π exchange NN interaction proportional to g _A ⁰ and g _A ². The resulting 2πγ exchange potential contains isospin-breaking components which reach up to about -2% of the corresponding isovector 2π exchange potential. With a value of only -17keV at r = m _π ^-1 = 1.4fm the charge-independence breaking central potential obtained here is negligibly small in comparison to the one generated by the isoscalar c₃ contact vertex. Our calculation confirms that the largest long-range isospin-violating NN potentials arise from the 2πγ exchange diagrams involving the large low-energy constants c ₄ ≃ - c ₃ ≃ 3.3GeV^-1 representing the important Δ(1232) dynamics.     

Similarity renormalization group evolution of chiral effective nucleon–nucleon potentials in the subtracted kernel method approach

Methods based on Wilson’s renormalization group have been successfully applied in the context of nuclear physics to analyze the scale dependence of effective nucleon–nucleon (NN) potentials, as well as to consistently integrate out the high-momentum components of phenomenological high-precision NN potentials in order to derive phase-shift equivalent softer forms, the so called V_low-k potentials. An alternative renormalization group approach that has been applied in this context is the similarity renormalization group (SRG), which is based on a series of continuous unitary transformations that evolve hamiltonians with a cutoff on energy differences. In this work we study the SRG evolution of a leading order (LO) chiral effective NN potential in the ¹S₀ channel derived within the framework of the subtracted kernel method (SKM), a renormalization scheme based on a subtracted scattering equation.     

Dibaryon concept for nuclear force and its experimental evidence

Numerous theoretical and experimental arguments are presented in favor of the generation of intermediate σ-dressed dibaryon in NN interaction at middle and short distances. We argue that this intermediate dibaryon can be responsible for the strong middle-range attraction and the short-distance repulsion in the NN interaction, and also for the short-range correlations in nuclei. The suggested mechanism for the σ dressing of the dibaryon is identical to that which explains the Roper-resonance structure, its dominant decay modes, and its extraordinary low mass. A similar transformation mechanism from the glue to the scalar field was discovered in J/ψ decays. The new experimental data on 2π production in the scalar-isoscalar channel in pn and pd collisions and in, particular, the very recent data on γγ correlations in pC and dC scattering in the GeV region seems to corroborate the existence of the σ-dressed dibaryon in two- and three-nucleon interactions.     

核子-核子碰撞截面对同位素标度参数α的同位旋效应

对两对重离子中心碰撞系统⁴⁰C+⁴⁰Ca 和⁶⁰Ca+⁴⁰Ca以及¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn反应中就同位素标度参数α对于核子-核子碰撞截面的同位旋效应进行了研究.计算结果表明α对同位旋相关核子-核子碰撞截面σ^med_{NN关键词： 同位旋效应 核子-核子截面 机理 同位素标度}     

Heavy-ion fusion cross-section calculations in classical microscopic equations of motion approach

Fusion cross-sections for¹⁶O +¹⁶O reaction earlier calculated in classical microscopic equations of motion approach, with Lennard-Jones form ofNN interaction potential are overestimated compared to the experimental data at lower energies. This large deviation was attributed to possible break down of classical approximations at lower energies. The aim of this paper is to show that this discrepancy was rather due to certain assumptions made in the specification of initial conditions; in particular due to neglect of Coulomb interaction between the colliding ions at far off distances. Use of Lennard-Jones potential is also critically examined.     

Folding model analysis of the nucleus–nucleus scattering based on Jacobi coordinates

This paper presents the results of scattering of ¹⁶O+²⁰⁹Bi interaction near the Coulomb barrier. The interaction potential between two nuclei is calculated using the double folding model with the effective nucleon–nucleon (NN) interaction. The calculations of the exchange part of the interaction were assumed to be of finite-range and the density dependence of the NN interaction is accounted for. Also the results are compared with the zero-range approximation. All these calculations are done using the wave functions of the two colliding nuclei in place of their density distributions. The wave functions are obtained by the D-dimensional wave equation using the hyperspherical calculations on the basis of Jacobi coordinates. The numerical results for the interaction potential and the differential scattering are in good agreement with the previous works.     

Arguments against one-boson-exchange models of nuclear forces and new mechanism for intermediate-and short-range nucleon-nucleon interaction

Arguments against the traditional Yukawa-type approach to NN intermediate-and shortrange interaction due to scalar-isoscalar and heavy-meson exchanges are presented. Instead of the Yukawa mechanism for intermediate-range attraction, some new approach based on the formation of a symmetric six-quark bag in the state |(0s)⁶[6]_X, L=0〉 dressed owing to strong coupling to π, σ, and ρ fields is suggested. This new mechanism offers a strong intermediate-range attraction, which replaces effective σ exchange (or excitation of two isobars in the intermediate state) in traditional force models. A similar mechanism with the production of a vector ρ meson in the intermediate six-quark state is expected to lead to a strong short-range spin-orbit nonlocal interaction in the NN system, which may resolve the long-standing puzzle of the spin-orbit force in baryons and in two-baryon systems. The effective interaction in the NN channel provided by the new mechanism will be enhanced significantly if the partial restoration of chiral symmetry is assumed to occur inside the six-quark symmetric bag. A simple illustrative model is developed that demonstrates clearly how well the suggested new mechanism can reproduce NN data. Strong interrelations have been shown to exist between the proposed microscopic model and one-component Moscow NN potential developed by the authors previously and also with some hybrid models and the one-term separable Tabakin potential. The new implications of the proposed model for nuclear physics are discussed.