Summary of Few Nucleon Working Group

Systems of Age 2 nucleons are arguably the most interesting area of application of hadronic effective field theories (EFTs), because of a great confluence of factors absent in A le 1 systems: the theoretical challenge of combining a momentum expansion and a resummation to produce bound states, and the abundance of experimental data. A ge 2 systems are also the newest playground for hadronic EFTs, even though Weinberg's seminal papers 1 date from almost ten years ago. The working group on few-body systems at Chiral Dynamics 2000 attests that this field is now viewed as part of mainstream Chiral Perturbation Theory (ChPT). Few-nucleon systems provide, more generally, a unique testing ground for the simple, traditional picture of the nucleus as a system of interacting nucleons. The nucleon-nucleon (NN) interaction, as revealed by pp and np scattering experiments and the deuteron's properties, has a very rich structure. In light nuclear systems, with only a few degrees of freedom, it is possible to obtain accurate solutions for a wide variety of nuclear properties directly from realistic models of the NN interaction. Within this deceptively simple picture, we can test our understanding of nuclear structure and dynamics over a wide range of energy, from the few keV of astrophysical relevance to the MeV regime of nuclear spectra to the tens to hundreds of MeV measured in nuclear response experiments. Through the advances in computational techniques and facilities, the last few years have witnessed dramatic progress in the theory of light nuclei, as well as a variety of intriguing new experimental results. Important advances have occurred in studies of the spectra and structure of light nuclei, hadronic scattering, the response of light nuclei to external probes, and electroweak reactions involving few-nucleon systems at very low energy. By the Chiral Dynamics Workshop in Mainz in the summer of 1997, the first successes of Weinberg's power counting had been seen in the description of the A = 2 system, and in the derivation of A = 3 forces 3 and currents. Nevertheless, some questions of consistency had also been raised: the roles of regularization, fine-tuning, and chiral-symmetry-breaking interactions were unclear. A discussion of some of these issues can be found in a review talk at that conference. Much progress has been made since then. The contributions to the working group, summarized here, reflect the current state of affairs in the rich interplay between theory--including the connection to the more conventional approach outlined above--and experiment