Heavy Baryon Chiral Perturbation Theory (HBChPT) is an effective quantum field theory used to describe the interactions of pions and nucleons/baryons. It is somewhat an extension of Chiral perturbation theory (ChPT) which just describes the low-energy interactions of pions. In a richer theory one would also like to describe the interactions of baryons with pions. A fully relativistic Lagrangian of nucleons is non-predictive as the quantum corrections, or loop diagrams can count as quantities and therefore do not describe higher-order corrections.
Because the baryons are much heavier than the pions, HBChPT rests on the use of a nonrelativistic description of baryons compared to that of the pions. Therefore, higher order terms in the HBChPT Lagrangian come in at higher orders of where is the baryon mass.
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