According to the SEP:
Wigners pioneering identification of the types of particles with irreducible unitary representations of the Poincare group has been exemplary until the present.
Regarding the question of whether Wigner had supplied a definition of particles, one must say that though he has contributed a valuable and fruitful classification of particles; his answer does not contribute very much to what a particle is and whether a given theory can be interpreted in terms of it.
Classically we think of an electron as a point particle which generates an electric field; one speaks of an electron and it's field - as though they are two ontologically different concepts; in a sense they are as one lacks all extension and the other does not.
The electron as a particle is generally regarded as prior, as is captured by how one speaks or writes of it.
But, in no instance do we see the bare electron without this field; so by what reasoning should we think of it as prior; other than the bias we have for atomic theories?
Let us try a little Gedanken experiment and eliminate the electron whilst leaving the field as it is.
Can we locate the classical position of the electron? Yes - by locating the position of the field with the most electric intensity.
Can we quantify the charge that this hypothetical particle has? Yes - because the relation to a field to its charge is deterministic.
Given that the particle can be determined - it's position and properties, from the field; and classically we can determine the field from the particle - so the two pictures are equivalent; then by what principle should we choose the particle picture over the field?