  Gamow vectors are generalized eigenvectors (kets) of self-adjoint
Hamiltonians with complex eigenvalues $(E_{R}\mp i\Gamma/2)$ describing
quasistable states. In the relativistic domain this leads to Poincar\'e
semigroup representations which are characterized by spin $j$ and by complex
invariant mass square
${\mathsf{s}}={\mathsf{s}}_{R}=(M_{R}-\frac{i}{2}\Gamma_{R})^{2}$. Relativistic
Gamow kets have all the properties required to describe relativistic resonances
and quasistable particles with resonance mass $M_{R}$ and lifetime
$\hbar/\Gamma_{R}$.
