  The Sivers mechanism for the single-spin asymmetry in unpolarized lepton
scattering from a transversely polarized nucleon is driven by the orbital
angular momentum carried by its quark and gluon constituents, combined with QCD
final-state interactions. Both quark and gluon mechanisms can generate such a
single-spin asymmetry, though only the quark mechanism can explain the small
single-spin asymmetry measured by the COMPASS collaboration on the deuteron,
suggesting the gluon mechanism is small relative to the quark mechanism. We
detail empirical studies through which the gluon and quark orbital angular
momentum contributions, quark-flavor by quark-flavor, can be elucidated.
