Effective Lagrangian
= Effective Lagrangian
An effective Lagrangian encodes the interactions of retained degrees of freedom after others have been removed, often by a <path integral>. For external energies below a massive field's excitation threshold, its effects admit a local expansion in external <momenta> divided by its mass. Matching <scattering amplitudes> fixes the coefficients of the resulting local operators; truncation has a specified order in that small ratio.