$\Delta(1232)$ resonance in the $\vec{\gamma}p\rightarrow p\pi^0$   reaction at threshold
  We calculate the neutral pion photoproduction on the proton near threshold in covariant baryon chiral perturbation theory, including the $\Delta(1232)$ resonance as an explicit degree of freedom, up to chiral order $p^{7/2}$ in the $\delta$ counting. We compare our results with recent low-energy data from the Mainz Microtron for angular distributions and photon asymmetries. The convergence of the chiral series of the covariant approach is found to improve substantially with the inclusion of the $\Delta(1232)$ resonance. 
A. N. Hiller Blin, T. Ledwig, M. J. Vicente Vacas
hep-ph