Reconciling the lattice background field method with nonrelativistic QED: Spinor case

We show that inconsistency between background field methods, which are relevant for lattice QCD spectroscopy, and effective field theory matching conditions, which are obtained from scattering amplitudes, can be resolved by augmenting nonrelativistic QED with operators related by the equations of motion. To determine the coefficients of such operators, we perform the nonrelativistic expansion of QED for a spin-half hadron including nonminimal electromagnetic couplings. As an effective field theory framework could provide a valuable tool to analyze lattice QCD correlation functions in external fields, we investigate whether nonrelativistic QED can be used to this end. We argue, however, that the most desirable approach is a hybrid one, which combines a relativistic hadron theory with operator selection based on nonrelativistic QED power counting. In this hybrid framework, new results are obtained for charged spin-half hadrons in uniform magnetic fields, including a proper treatment of Landau levels both in infinite volume and on a torus.