Types of quantum field thories
However, even so, abstractly the formalization of the concept of particle within QFTs is somewhat subtle.
If the quantum field theory is on Minkowski space and comes with a Hilbert space of states on which thus the Poincare group of translations, rotations and boosts in Minkowski space acts, the massive particle excitations of the theory can be found in the discrete spectrum of the time translation operator as the irreducible unitary representations of the Poincare group.
Often, however, QFTs are considered as quantizations of given Lagrangians. In these cases one often identifies their particle content with that explicitly encoded by the Lagrangian. Notably when that ariseses from second quantization of some 1-dimensional sigma-model, the particles of the theory are those described by these sigma-models.
Chapter VI of