Virtual photons in baryon chiral perturbation theory Original Research Article

We construct the general Lagrangian for relativistic and heavy baryon chiral perturbation theory with virtual photons to fourth order. We work out the electromagnetic and strong isospin violating contributions to the nucleon self-energy, the nucleon mass and the scalar form factor of the nucleon. Electromagnetic effects for the shift to the Cheng-Dashen point can be as large as 2 MeV We also discuss the corrections to Weinbergʹs prediction for the scattering length difference a(π0p) − a(π0n) and show that they are small.