Singularity of Feynman propagator and Cutkosky's cutting rules
  We improve on Cutkosky's cutting rules which is used to calculate the contribution of the singularities of Feynman propagators to Feynman amplitude. The correctness of the improved cutting rules is verified by the calculations of the conventional loop momentum integral algorithm. We also find that only the process of the intermediate on shell particles that is same as real physical process can exist in quantum vacuum. 
Yong Zhou