2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/47713We discuss the positivity of the hadron density matrix in QCD. This basic property is shown to be preserved by QCD evolution, provided the relation $|ΔP_{ij}(z)| \leq P_{ij}$ is valid for all kernels for $z < 1$, and the usual "+" prescription is used. We comment on the positivity restrictions for the choice of the NLO factorization scheme for the evolution of the spin-dependent parton distributions.6 pages, LateX, 1 figureHigh Energy Physics - Phenomenologytext