Perturbation theory with unstable fundamental fields
  The difficulties of perturbation theory associated with unstable fundamental fields (such as the lack of exact gauge invariance in each order) are cured if one constructs perturbative expansion directly for probabilities interpreted as distributions in kinematic variables. Such an expansion is made possible by the powerful method of non-Euclidean asymptotic operation [hep-ph/9703424]. 
Fyodor V. Tkachov