Nucleon-nucleon interactions in the large-Nc expansion

One of the main goals in nuclear physics is to relate the phenomenologically highly successful nucleon interactions to the underlying theory of QCD. Currently, fully nonperturbative calculations, such as in lattice QCD, still pose significant challenges. Effective field theory (EFT) methods have been used to establish connections between QCD and nucleon-nucleon interactions based on symmetries. I will describe how considering QCD in the limit of the number of colors Nc being large can further constrain these EFT interactions. In the usual symmetry-preserving sector, these constraints show general agreement with the results obtained from fits to the available scattering data. The large-N_c constraints are particularly valuable for cases in which limited amounts of data exist (if at all), such as for nucleon-nucleon interactions that violate parity (P) and time reversal invariance (T). These types of symmetry-violating interactions can be used as precision tests of our understanding of the Standard Model as well as in searches for physics beyond the Standard Model. I will discuss how the large-N_c expansion helps to delineate the terms that should be most important in phenomenological applications for both P-violating and T-violating interactions.