= Beam spin polarization
{title2=$P=(N_+-N_-)/(N_++N_-)$}
A beam's unequal population of <spin> projections along a specified axis. For spin-one-half particles the displayed <longitudinal polarization> uses populations parallel and antiparallel to the <momentum>. An unpolarized beam has equal populations and requires an initial-spin average in a <cross section>; a polarized beam weights the corresponding <helicity> amplitudes differently. This differs from <photon> linear polarization.
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