Electroweak Probes and Nuclear Effective Field the - Ory

There is growing interest in nuclear physics applications of effective field theory. I give a brief account of some of the latest developments in this area. I also describe interplay between this new approach and the traditional nuclear physics approach. As is well known, the description of nuclear structure based on the phenomenological potential picture has been extremely successful [1]. In this picture, nuclear responses to external probes are given by one-body impulse approximation terms and exchange-current terms (usually two-body terms) acting on nuclear wave functions, with the exchange currents derived from a one-boson exchange model. In a modern realization of this approach [2], the vertices characterizing relevant Feynman diagrams are determined with the use of the low-energy theorems and current algebra. We may refer to this type of formalism as SNPA, the standard nuclear physics approach. SNPA has been used extensively for nuclear electroweak processes, and the reported good agreement between theory and experiment suggests the basic soundness of SNPA [1]. Meanwhile, one of the major challenges in nuclear physics today is to establish a connection between nuclear dynamics and the fundamental QCD. Effective field theory (EFT) [3, 4] is considered to offer a natural and useful framework for this purpose. In this talk I first give a highly abridged account of EFT as applied to nuclear physics. I then discuss what may be called a " symbiotic " relation between EFT and SNPA. The basic idea underlying EFT is quite simple. In describing phenomena characterized by a typical energy-momentum scale Q, we need not explicitly include in our Lagrangian those degrees of freedom which are characterized by energy-momentum scales much larger than Q. Then, introducing a cutoff scale Λ that is substantially larger than Q, we may classify our fields (generically denoted by Φ) into two parts: a high-energy part Φ H and a low-energy part Φ L. Integrating out Φ H leads to an effective Lagrangian that only involves Φ L ; thus the original Lagrangian L and the effective Lagrangian L eff are related to each