We formulate a new power-counting scheme for a chiral effective-field theory of nucleons, pions, and $\Delta \mathrm{s}.$ This extends chiral perturbation theory into the $\Delta$-resonance region. We calculate nucleon Compton scattering up to next-to-leading order in this theory. The resultant description of existing $\gamma p$ cross-section data is very good for photon energies up to about 300 MeV. We also find reasonable numbers for the spin-independent polarizabilities ${\alpha }_p$ and ${\beta }_p.$

• Received 4 December 2002

DOI:https://doi.org/10.1103/PhysRevC.67.055202