Particles can be seen as "Unitary irreducible representations of the Poincare' group" using the Casimir of the representations: $m$ and $j$. Now, is it possible to generalize this procedure in order to classify all the "Unitary irreducible representations of the Diffeomorphism group"? If it's possible, does this give a generalization of the concept of particle? In both cases the group is acting on the space-time.