2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/182664The Sivers mechanism gives rise to a non-zero average transverse momentum of partons inside a transversely polarized target. According to a sum rule of Burkardt this transverse momentum vanishes when summing over all partons. We explicitly check the Burkardt sum rule by means of one-loop calculations for a quark target in perturbative QCD and for a diquark spectator model of the nucleon. In both cases the sum rule is fulfilled.final version, to appear in Phys. Lett. BHigh Energy Physics - Phenomenologytext