Ghost poles and chiral symmetry in $\pi N$ scattering
  We solve the Schwinger-Dyson equation for the nucleon propagator in the vacuum using $\pi$, $\sigma$, and $\omega$ mesons. For bare interaction vertices (Hartree-Fock approximation) we obtain a pair of complex-conjugated poles (ghost poles) and for vertices dressed by phenomenological form-factors these ghosts disappeared. We use these two different approaches to evaluate the scattering lengths and the phase shifts for the $\pi N$ scattering at threshold. Our results show that only when the form factors are present is possible to obtain a good agreement with the low-energy observables. 
C.A. da Rocha, G. Krein, and L. Wilets