Review and concrete description of the irreducible unitary representations of the universal cover of the complexified Poincaré group

In this paper, we give a pedagogical presentation of the irreducible unitary representations of 4 Spin(4, ), that is, of the universal cover of the complexified Poincaré group 4 SO(4, ). These representations were first investigated by Roffman in 1967. We provide a modern formulation of his results together with some facts from the general Wigner-Mackey theory which are relevant in this context. Moreover, we discuss different ways to realize these representations and, in the case of a non-zero "complex mass", we give a detailed construction of a more explicit realization. This explicit realization parallels and extends the one used in the classical Wigner case of 4 Spin0(1, 3). Our analysis is motivated by the interest in the Euclidean formulation of Fermionic theories.