Complex-mass scheme and perturbative unitarity
  We derive cutting rules for loop integrals containing propagators with complex masses. Using a field-theoretical model of a heavy vector boson interacting with a light fermion, we demonstrate that the complex-mass scheme respects unitarity order by order in a perturbative expansion provided that the renormalized coupling constant remains real. 
T. Bauer, J. Gegelia, G. Japaridze and S. Scherer
hep-ph hep-th