When one takes into account the transverse momenta of the constituents in a polarized proton, there exists a kinematic, "trigger-bias," effect in the formulation of the QCD-based hard-scattering model which can lead to single-spin production asymmetries. It seems convenient to represent the coherent spin-orbit forces in a polarized proton by defining an asymmetry in the transversemomentum distribution of the fundamental constituents. It may then be possible to organize the hard-scattering model so that the kinematic constraints of hard 2-2 scattering provide the leading contribution at large transverse momentum to asymmetries of the type ANdo(hpt+jet+x), ANd0(hpr + "~''xx), where pt denotes a transversely polarized proton and "a" represents any spinless meson composed of light quarks. This approach provides testable relationships between different asymmetries.

The extraction of a scaling law of the form [ ( p + + p 2 ) / p p T ] ~N -g (~T ) (where from data on a single-spin production asymmetry such as A,\rdo(ppr +sox) at large transverse momentum can be used to argue for an underlying "hard-scQteringn mechanism. Data from the upcoming Fermilab polarized beam experiment (E-704) can be used to test the scaling hypothesis.In a parity-conserving theory there can exist a nontrivial single-spin production asymmetry where the polarized proton's spin is transverse to its direction of motion and the momentum of the produced pion is normal to both the proton spin and momentum. For large-transverse-momentum production, it is interesting to test whether such a single-spin production asymmetries as A , v d u ( p p y -rrox) or A , d~( p p .~ +jet X ) can be understood within the overall context of the Q C D hard-scattering model. Some theoretical studies' have been used to claim that such single-spin observables must strictly vanish in the hard-scattering regime because of quark-helicity conservation and, hence, that any nonvanishing asymmetry is to be taken as an indication that the data are not yet in a kinematic region where Q C D perturbation theory can be considered valid.It is possible to refute this argument. There exists a body of theoretical ~o r k~,~ that has questioned the hypothesis that the vanishing of single-spin asymmetries is necessary in the hard-scattering regime by pointing out that "hadronic" masses rather than "current-quark" masses can enter into the calculation of such observables in QCD. In order to demonstrate the underlying principle, in a recent paper2 I showed that when one accepts the possibility of asymmetries in the intrinsic-kT distribution of constituents within a polarized proton, there exists a well-understood kinematic, "trigger-bias," effect in the formulation of the QCD-based hard-scattering model that can lead to significant single-spin production symmetries at large transverse momentum. This simple approach provides a hypothetical framework for understanding such observables as recognizable "higher-twist" components of the basic theory. In fact, the model advocated in Ref. 2 can be considered an extremely naive manifestation of the general theoretical picture in that the intrinsic-k, distribution that gives the asymmetry is controlled by a hadronic mass scale and need not vanish as m, -0.However, it would be nice to bypass specific theoretical models and to formulate an explicitly experimental test for the presence of hard scattering. One useful criterion for defining the kinematic regime where hard-scattering dynamics are appropriate involves the application of scaling laws.4 The basic assumption for the formulation of scaling laws is that the separation of hard-scattering and soft dynamics can be done in a manner only weakly dependent on. the choice of factorization scale. When hard-scattering cross sections are "factored out" the remainder must depend only on scale-invariant kinematic variables. When this is done in the familiar examp...

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