Nothing anomalous about two loop HBCPT: Consistency with the LET for spin dependent Compton scattering

The leading order contributions of processes involving anomalous pion-photon vertices to forward spin-dependent Compton scattering from nucleons are considered in heavy-baryon chiral perturbation theory. These all involve the exchange of three pions between one or two photons and the nucleon, and hence are two-loop processes. We find that the sum of these processes vanishes in the manner predicted by the low energy theorem of Low, Gell-Mann and Goldberger as the photon energy goes to zero. This provides the first consistency test of two-loop HBCPT.

vertex function: (pi photon)
photon nucleon: Compton scattering
exchange: (3pi)
spin dependence
perturbation theory: chiral
baryon: heavy
Feynman graph: higher-order
low-energy theorem

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