  Lorentz boosts on particles with spin and momentum degrees of freedom induce
momentum-dependent rotations. Since, in general, different particles have
different momenta, the transformation on the whole state is not a
representation of the rotation group. Here we identify the group that acts on a
two-particle system and, for the case when the momenta of the particles are
correlated, find invariant subspaces that have interesting properties for
quantum information processes in relativistic scenarios. A basis of states is
proposed for the study of transformations of spin states under Lorentz boosts,
which is a good candidate for building quantum communication protocols in
relativistic scenarios.
