Density matrix and polarized particle physics

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Abstract
Applying the density matrix in the context of helicity amplitude techniques for using Feynman perturbation theory, some straightforward and general demonstrations of polarization properties in particle collisions are presented. Among these properties, by using longitudinal modes and classifying the transverse polarizations as being either entirely within the scatter plane or perpendicular to the scatter plane, certain asymmetries are shown to be sensitive to higher-order corrections; double-spin asymmetries with only one transverse polarization are also shown to vanish in many situations, while related asymmetries which were zero in the single-spin case obtain nonzero Born level contributions. Such results may be interpreted in a similar fashion whether the particles be photons, gluons, quarks, or leptons in the initial or final state.