  Weak decays of beauty baryons like Lambda_b into Lambda V(J^P=1^-), where the
produced resonances are polarized, offer interesting opportunity to perform
tests of Time-Reversal Invariance. This paper emphasizes the particular role of
the resonance polarization-vectors and their physical properties by symmetry
transformations. In particular, it is shown that the normal component of a
polarization-vector, as defined in the Jackson's frame, is Lorentz invariant
and could get large values, notably in the case of J/psi production.
