  We compute, for massive particles, the explicit Wigner rotations of
one-particle states for arbitrary Lorentz transformations; and the explicit
Hermitian generators of the infinite-dimensional unitary representation. For a
pair of spin 1/2 particles, Einstein-Podolsky-Rosen-Bell entangled states and
their behaviour under the Lorentz group are analysed in the context of quantum
field theory. Group theoretical considerations suggest a convenient definition
of the Bell states which is slightly different from the conventional
assignment. The behaviour of Bell states under arbitrary Lorentz
transformations can then be described succinctly. Reduced density matrices
applicable to identical particles are defined through Yang's prescription. The
von Neumann entropy of each of the reduced density matrix is Lorentz invariant;
and its relevance as a measure of entanglement is discussed, and illustrated
with an explicit example. A regularization of the entropy in terms of
generalized zeta functions is also suggested.
