In quantum field theory, the Wick rotation provides the standard technique of computing Feynman diagrams by means of Euclidean propagators.
Let us suppose that quantum fields in an interaction zone are really Euclidean. In contrast with the well-known Euclidean field theory dealing with the Wightman and Schwinger functions of free quantum fields, we address complete Green's functions of interacting fields, i.e., causal forms on the Borchers algebra of quantum fields. They are the
Laplace transform of the Euclidean states obeying a certain condition.
If Euclidean states of a quantum field system, e.g., quarks do not satisfy this condition, this system fails to possess Green's functions and, consequently, the S-matrix. One therefore may conclude that it is not observed in the Minkowski space.
G.Sardanashvily, arXiv: hep-th/0511111