The NRQED lagrangian at order 1/M^4

Understand

The parity and time-reversal invariant effective lagrangian for a heavy fermion interacting with an abelian gauge field, i.e., NRQED, is constructed through order $1/M^4$.

• The implementation of Lorentz invariance in the effective theory becomes nontrivial at this order, and a complete solution for Wilson coefficient constraints is obtained.
• Matching conditions in the one-fermion sector are presented in terms of form factors and two-photon matrix elements of the nucleon.
• The extension of NRQED to describe interactions of the heavy fermion with a light fermion is introduced.