We show that the Bakamjian-Thomas construction for relativistic direct interactions can be extended to describe resonances by irreducible representations of the causal Poincaré semigroup. These representations are generated by a Poincaré algebra that incorporates interactions and are characterized by the complex pole position and spin of the resonance.
Sujeev Wickramasekara. "Gamow Vectors in the Bakamjian-Thomas Construction." J. Geom. Symmetry Phys. 11 67 - 75, 2008. https://doi.org/10.7546/jgsp-11-2008-67-75