Conference Discussion of the Nuclear Force

Discussion of the nuclear force, lead by a round table consisting of T. Cohen, E. Epelbaum, R. Machleidt, and F. Gross (chair). After an invited talk by Machleidt, published elsewhere in these proceedings, brief remarks are made by Epelbaum, Cohen, and Gross, followed by discussion from the floor moderated by the chair. The chair asked the round table and the participants to focus on the following issues: (i) What does each approach (chiral effective field theory, large Nc, and relativistic phenomenology) contribute to our knowledge of the nuclear force? Do we need them all? Is any one transcendent? (ii) How important for applications (few body, nuclear structure, EMC effect, for example) are precise fits to the NN data below 350 MeV? How precise do these fits have to be? (iii) Can we learn anything about nonperturbative QCD from these studies of the nuclear force? The discussion presented here is based on a video recording made at the conference and transcribed afterward.Comment: Discussion at the 21st European Conference on Few Body Problems (EFP21) held at Salamanca, Spain, 30 Aug - 3 Sept 201

Chiral Symmetry and the Nucleon-Nucleon Interaction

We review how nuclear forces emerge from low-energy quantum chromodynamics (QCD) via chiral effective field theory (EFT). During the past two decades, this approach has evolved into a powerful tool to derive nuclear two- and many-body forces in a systematic and model-independent way. We then focus on the nucleon-nucleon (N N) interaction and show in detail how, governed by chiral symmetry, the long- and intermediate-range of the N N potential builds up order by order. We proceed up to sixth order in small momenta, where convergence is achieved. The final result allows for a full assessment of the validity of the chiral EFT approach to the N N interaction

Impact of three-body forces on elastic nucleon-nucleus scattering observables

none6siBackground: In a previous series of papers we investigated the domain of applicability of chiral potentials to the construction of a microscopic optical potential (OP) for elastic nucleon-nucleus scattering. The OP was derived at the first order of the spectator expansion of the Watson multiple scattering theory and its final expression was a folding integral between the nucleon-nucleon (NN) t matrix and the nuclear density of the target. In the calculations NN and three-nucleon (3N) chiral interactions were used for the target density and only the NN interaction for the NN t matrix. Purpose: The purpose of this work is to achieve another step towards the calculation of a more consistent OP introducing the 3N force also in the dynamic part of the OP. Methods: The full treatment of the 3N interaction is beyond our present capabilities. Thus, in the present work it is approximated with a density dependent NN interaction obtained after the averaging over the Fermi sphere. In practice, in our model the 3N force acts as a medium correction of the bare NN interaction used to calculate the t matrix. Even if the 3N force is treated in an approximate way, this method naturally extends our previous model of the OP and allows a direct comparison of our present and previous results. Results: We consider as case studies the elastic scattering of nucleons off 12C and 16O. We present results for the differential cross section and the spin observables for different values of the projectile energy. From the comparison with the experimental data and with the results of our previous model we assess the importance of the 3N interaction in the dynamic part of the OP. Conclusions: Our analysis indicates that the contribution of the 3N force in the t matrix is small for the differential cross section and it is sizable for the spin observables, in particular, for the analyzing power. We find that the two-pion exchange term is the major contributor to the 3N force. A chiral expansion order-by-order analysis of the scattering observables confirms the convergence of our results at the next-to-next-to-next-to-leading-order, as already established in our previous work.openVorabbi, Matteo; Gennari, Michael; Finelli, Paolo; Giusti, Carlotta; Navrátil, Petr; Machleidt, RuprechtVorabbi, Matteo; Gennari, Michael; Finelli, Paolo; Giusti, Carlotta; Navrátil, Petr; Machleidt, Ruprech

Family of chiral two- plus three-nucleon interactions for accurate nuclear structure studies

We present a family of nucleon-nucleon (NN) plus three-nucleon (3N) interactions up to N3LO in the chiral expansion that provides an accurate ab initio description of ground-state energies and charge radii up to the medium-mass regime with quantified theory uncertainties. Starting from the NN interactions proposed by Entem, Machleidt and Nosyk, we construct 3N interactions with consistent chiral order, non-local regulator, and cutoff value and explore the dependence of nuclear observables over a range of mass numbers on the 3N low-energy constants. By fixing these constants using the 3-H and 16-O ground-state energies, we obtain interactions that robustly reproduce experimental energies and radii for large range from p-shell nuclei to the nickel isotopic chain and resolve many of the deficiencies of previous interactions. Based on the order-by-order convergence and the cutoff dependence of nuclear observables, we assess the uncertainties due the interaction, which yield a significant contribution to the total theory uncertainty.Comment: 6 pages, 5 figures, 1 tabl