Microcausality in strongly interacting fields
We study the properties of strongly interacting massive quantum fields in space-time as resulting from a parametric decay of the fields with a large decay width $\gamma$. The resulting imaginary part of the retarded and advanced propagators in this case is of Lorentzian form and the theory conserves microcausality, i.e. the commutator between the fields vanishes for space-like distances in space-time. However, when considering separately space-like and time-like components of the spectral function in momentum space we find microcausality to be violated for each component separately. This implies that the modeling of effective field theories for strongly interacting systems has to be considered with great care and restrictions to time-like four momenta in case of broad spectral functions have to be ruled out. Furthermore, when employing effective propagators with a width $\gamma({\bf p}^2)$ depending explicitly on three-momentum ${\bf p}$ the commutator of the fields no longer vanishes for $r>t$ since the related field theory becomes nonlocal and violates microcausality.