Derivation of O(q⁴) Effective Lagrangian in the Presence of External Fields Directly Within Heavy Baryon Chiral Perturbation Theory

This is a progress report on the extension of the analysis of hep-ph/9909498 to constructing a complete list of O(q^4) terms in the presence of external fields, directly within Heavy Baryon Chiral Perturbation Theory (HBChPT) {\it without having to first construct the relativistic BChPT Lagrangian and then carry out the ${1\over\rm m}$-reduction}. In addition to a phase rule to implement all symmetries including charge conjugation invariance directly at the nonrelativistic level, generalized Jacobi identities, curvature relation (that relates the commutator of two covariant derivatives to a linear combination of the traceless and isosinglet field strengths and the commutator of two axial vector building blocks), Schouten's identity and the relationship between the antisymmetrized covariant derivative - axial-(building block) vector commutator and another traceless field strength, are used to ensure linear independence of the terms and their low energy coupling constants. We first construct O($q^4$) terms for off-shell nucleons, and then perform the on-shell reduction, again within HBChPT.