  Single-spin asymmetries observed in polarised deep-inelastic scattering are
important probes of hadron structure. The Sivers asymmetry has been the focus
of much attention in QCD phenomenology and is yet to be understood at the quark
level. In this Letter, we present a lattice QCD calculation of the spatial
distribution of a colour-Lorentz force acting on the struck quark in a proton.
We determine a spin-independent confining force, as well as spin-dependent
force distributions with local forces on the order of 3 GeV/fm. These
distributions offer a complementary picture of the Sivers asymmetry in
transversely polarised deep-inelastic scattering.
