Representations of the Lorentz group corresponding to unstable particles

The irreducible nonunitary representations of the inhomogeneous Lorentz group which satisfy the condition that the complex energy-momentum 4 vectors be transformable into a rest frame by a real Lorentz rotation are found. By explicit construction of wave packets in configuration space it is seen that the vectors of the representation space have a natural interpretation as states of unstable particles. The new representations arise in the study of complex singularities associated with resonance poles in analytically continued scattering amplitudes.