Effective field theories (EFTs) are a powerful means of analyzing physical processes for which we are not able to compute observables entirely from first principles. Situations for which EFT is ideally suited include those wherein non-perturbative strong interaction effects are important, such as low-energy interactions of systems involving light quarks (e.g., nucleons and pions), the decays of mesons that contain one heavy quark and one light quark; or jet observables. EFT methods can also be applied to describe the possible low-energy effects of physics beyond the standard model when we don't know the full theory at high scales but can classify possible low-energy effects based on symmetry considerations. In each application, the usefulness of EFT relies on the presence of a separation of scales, such as the pion mass (~ 140 MeV) compared to the scale of chiral symmetry breaking (~ 1 GeV). The presence of a scale separation allows us to systematically classify operators in terms of powers of scale ratios (small over large), obtaining a complete and self-consistent description of a particular process at a given order in the appropriate scale ratio.

NPAC theorists are been applying EFT methods to a variety of problems. We have used chiral perturbation theory to study the electromagnetic structure and weak interactions of nucleons and pions; nucleon-nucleon effective field theory to characterize parity-violating weak interactions in few-nucleon systems; heavy Majorana particle exchange contributions to neutrinoless double beta-decay (see Neutrino Properties); general EFT methods to study the implications of the scale of neutrino mass for neutrino magnetic moments and interactions (see Neutrino Properties); and general EFT methods to analyze possible modifications of Higgs boson production and decays at the Large Hadron Collider and possible e+e- linear collider (see Higgs Boson). We are continuing to apply EFT methods in all of these cases while extending them to other tests of fundamental symmetries in nuclei (see Symmetries in Nuclei).