entropy as a Lorentz invariant characterization of entanglement.

We compute, for massive particles, the explicit Wigner rotations of one-particle states; and the explicit generators of the unitary representation of the Lorentz group. Unitary representations of the Poincare group acting on physical states are founded upon Wigner’s seminal work[@wigner]; but the explicit expressions of Wigner rotations for massive particles have been computed, with some difficulty by direct matrix multiplication, only for rotations and boosts considered separately[@ohnuki]. Our derivation, carried out in Section II, is somewhat simpler, and it permits the explicit general result for arbitrary infinitesimal Lorentz transformations to be stated as in Eq.(\[e:wrg\]). Moreover, with the infinitesimal Wigner angle at hand, the explicit infinite-dimensional Hermitian generators of the unitary representation can be worked out, as in Section III. Wigner rotation for a finite general Lorentz transformation is slightly more complicated, but the explicit form is also listed in Appendix A.

As basic entangled states, Bell states figure prominently in the literature on quantum information science and in quantum computational schemes. Their behaviour under Lorentz transformations is therefore of interest and importance. In this article we focus on Bell states of two spin $\frac{1}{2}$ massive fermions. Under Lorentz transformations, each one-particle 