Complex mass definition and the concept of continuous mass
  Propagators of unstable particles are considered in the spectral representation which naturally follows from the concept of continuous mass. The spectral functions are found with the help of the most general formal and physical assumptions. Dressed propagators of unstable scalar, vector, and spinor fields are derived in an analytical way for a variant of parameter space.   The structure of the propagators is in a correspondence with the complex mass scheme. 
Vladimir Kuksa
hep-ph