Field redefinitions at finite density

The apparent dependence of nuclear matter observables on off-shell properti es of the two-nucleon potential is re-examined in the context of effective field theory (EFT). Finite density (thermodynamic) observables are invarian t under field redefinitions, which extends the well-known theorem about the invariance of S-matrix elements. Simple examples demonstrate how field red efinitions can shift contributions between purely off-shell two-body intera ctions and many-body forces, leaving both scattering and finite-density obs ervables unchanged. If only the transformed two-body potentials are kept, h owever, the nuclear matter binding curves will depend on the off-shell part (generating "Coester bands"), The correspondence between field redefinitio ns and unitary transformations, which have traditionally been used to gener ate "phase-equivalent" nucleon-nucleon potentials, is also demonstrated. (C ) 2001 Elsevier Science B.V. All rights reserved.