  We consider two spacelike separated Dirac particles and construct five
invariants under the spinor representations of the local proper orthochronous
Lorentz groups. All of the constructed Lorentz invariants are identically zero
for product states. The behaviour of the Lorentz invariants under local unitary
evolutions that act unitarily on any subspace with fixed particle momenta is
studied. All of the Lorentz invariants have invariant absolute values on such
subspaces if the evolutions are generated by local zero-mass Dirac
Hamiltonians. Some of them also for the case of nonzero-mass. Therefore, they
are considered potential candidates for describing spinor entanglement of two
Dirac particles, with either zero or arbitrary mass. Furthermore, their
relations to the Wootters concurrence is investigated and their representations
in the Foldy-Wouthuysen picture is given.
