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We present a new interpretation for reduced density matrices of secondaryvariables in relativistic systems via an analysis of Wigner's method toconstruct the irreducible unitary representations of the Poincar\'e group. Weargue that the usual partial trace method used to obtain spin reduced matricesis not fully rigorous, however, employing our interpretation, similar effectivereduced density matrices can be constructed. In addition, we show that ourproposal is more useful than the usual one since we are not restricted only tothe reduced density matrices that could be obtained by the ordinary partialtrace method.

A new view on spin reduced density matrix for relativistic particles