Valence spin observables in hard-scattering QCD and the measurement of the gluon spin density

To test the validity of the QCD-based hard-scattering model for spin-dependent measurements, it is useful to define observables which depend only on the valence-quark spin distributions. If theory proves an effective guide to these ‘‘valence’’ observables, the valence-quark contribution to other spin-dependent large-${\mathit{p}}_{\mathit{T}}$ processes can be specified with similar accuracy. We can then formulate large-${\mathit{p}}_{\mathit{T}}$ measurements which determine the gluon spin density in a polarized proton.