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Proposed a new method which is suitable for systematic calculations in few-body systems, we call it the Jacobi-coordinate-basis antisymmetrized molecular dynamics (JAMD). We show good abilities of the method by demonstrating some examples in atoms, molecules, nuclei, and baryons systems.

Three– and four–nucleon systems are described using the hyperspherical harmonic (HH) method. Bound and scattering states are expanded in the HH basis and the corresponding binding energies and S-matrices are obtained using a variational principle. Modern nucleon-nucleon potentials plus three-nucleon interactions are considered. The calculated quantities as binding energies, cross section and polarization observables are accurate at the level of 1% or better.

In this review, we present the recent advances for calculations of the reactions NN → NNπ using chiral effective field theory χEFT. Discussed are the next-to-next-to leading order (N²LO) loop contributions with nucleon and Delta-isobar for near threshold s-wave pion-production. Results of recent experimental pion-production data for energies close to the threshold are analyzed. Several particular applications are discussed: (i) it is shown how the measured charge symmetry (CS) violating pion-production reaction can be used to extract the strong interaction contribution to the proton–neutron mass difference; (ii) the role of NN → NNπ for the extraction of the pion–nucleon scattering lengths from pionic atoms data is illuminated.

In this paper, the role of three-nucleon forces in ab initio calculations of nuclear systems is investigated. The difference between genuine and induced many-nucleon forces is emphasized. Induced forces arise in the process of solving the nuclear many-body problem as technical intermediaries toward calculationally converged results. Genuine forces make up the Hamiltonian. They represent the chosen underlying dynamics. The hierarchy of contributions arising from genuine two-, three- and many-nucleon forces is discussed. Signals for the need of the inclusion of genuine three-nucleon forces are studied in nuclear systems, technically best under control, especially in three-nucleon and four-nucleon systems. Genuine three-nucleon forces are important for details in the description of some observables. Their contributions to observables are small on the scale set by two-nucleon forces.

Systems composed of a few nucleons are subject of experimental studies for many years with the aim to gain precise information on the interaction between nucleons. Intermediate energies, below the threshold for pion production, deserve special attention: it is a region where comparison with exact theoretical calculations is possible, while the sensitivity to various aspects of interaction, like subtle effects of the dynamics beyond the pairwise nucleon-nucleon force, is significant. In addition to differential cross section, the observables related to nuclear polarization are studied, like vector and tensor analyzing powers, spin-correlation coefficients or polarization transfer coefficients. A brief survey of results of recent studies in few-nucleon systems is given.

We analyze pion production from nucleon-deuteron collisions, with the outgoing three-nucleon system in bound state. Potentially, these reactions could be used to dissection three-nucleon force diagrams, which lead to three-nucleon potential operators. The experimental data, from threshold up to the Δ resonance, are compared with calculations using accurate nuclear wavefunctions. Pion production amplitudes are obtained through matrix elements involving pion-nucleon rescattering mechanisms in S- and P-waves. We assume the hypothesis that S-wave rescattering includes an isoscalar contribution which is generally suppressed for low-energy pion-nucleon scattering, but is enhanced for pion production because the kinematical regime is different, with involvment of high-momenta contributions. P-wave rescattering includes also explicitly the Δ degrees of freedom. Initial-state interactions (ISI) between the proton and the deuteron have sizable effects on the spin-averaged observables. These ISI effects become important for spin observables such as the deuteron tensor analyzing powers T₂₀. For spin observables involving interference terms amongst the various helicity amplitudes, such as for the nucleon vector analyzing power A_y, ISI effects are very important.

Three– and four–nucleon systems are described using the hyperspherical harmonic (HH) method. Bound and scattering states are expanded in the HH basis and the corresponding binding energies and S-matrices are obtained using a variational principle. Modern nucleon-nucleon potentials plus three-nucleon interactions are considered. The calculated quantities as binding energies, cross section and polarization observables are accurate at the level of 1% or better.

We summarize the findings of our Working Group, which discussed progress in the understanding of Chiral Dynamics in the A = 2, 3, and 4 systems over the last three years. We also identify key unresolved theoretical and experimental questions in this field.