Title: UNIFYING THE MECHANISMS FOR SINGLE SPIN ASYMMETRIES IN HARD PROCESSES.

Using lepton-pair production in hadron-hadron collisions as an example, we explore the relation between two well-known mechanisms for single-transverse-spin asymmetries in hard processes: twist-three quark-gluon correlations when the pair's transverse momentum is large, q{sub perpendicular}>>{lambda}{sub QCD}, and time-reversal-odd and transverse-momentum-dependent parton distributions when q{sub perpendicular} is much less than the pair's mass. We find that, although the two mechanisms each have their own domain of validity, they describe the same physics in the kinematic region where they overlap. This unifies the two mechanisms and imposes an important constraint on phenomenological studies.

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 transverse-momentum distribution of the fundamental constituents. It may then be possible to organize the hard-scattering model so that the kinematic constraints of hard 2{r arrow}2 scattering provide the leading contribution at large transverse momentum to asymmetries of the type {ital A}{sub {ital N}}d{sigma}(hp{sub {up arrow}}{rmore » arrow}jet+{ital x}), {ital A}{sub {ital N}}d{sigma}(hp{sub {up arrow}}{r arrow} {pi}''x), where {ital p}{sub {up arrow}} denotes a transversely polarized proton and {pi}'' represents any spinless meson composed of light quarks. This approach provides testable relationships between different asymmetries.« less

It is usually assumed--following the parton model--that the leading-twist structure functions measured in deep inelastic lepton-proton scattering are simply the probability distributions for finding quarks and gluons in the target nucleon. In fact, gluon exchange between the outgoing quarks and the target spectators effects the leading-twist structure functions in a profound way, leading to diffractive leptoproduction processes, shadowing and antishadowing of nuclear structure functions, and target spin asymmetries, physics not incorporated in the light-front wavefunctions of the target computed in isolation. In particular, final-state interactions from gluon exchange lead to single-spin asymmetries in semi-inclusive deep inelastic lepton-proton scattering which aremore » not power-law suppressed in the Bjorken limit. The shadowing and antishadowing of nuclear structure functions in the Gribov-Glauber picture is due respectively to the destructive and constructive interference of amplitudes arising from the multiple-scattering of quarks in the nucleus. The effective quark-nucleon scattering amplitude includes Pomeron and Odderon contributions from multi-gluon exchange as well as Reggeon quark-exchange contributions. Part of the anomalous NuTeV result for sin{sup 2} {theta}{sub W} could be due to the non-universality of nuclear antishadowing for charged and neutral currents. Detailed measurements of the nuclear dependence of individual quark structure functions are thus needed to establish the distinctive phenomenology of shadowing and antishadowing and to make the NuTeV results definitive. I also discuss diffraction dissociation as a tool for resolving hadron substructure Fock state by Fock state and for producing leading heavy quark systems.« less

The extraction of a scaling law of the form (({ital p}{sub {ital T}}{sup 2}+{mu}{sup 2})/{mu}{ital p}{sub {ital T}}){ital A}{sub {ital N}}{congruent}{ital g}({ital x}{sub {ital T}}) (where {ital x}{sub {ital T}}=2{ital p}{sub {ital T}}/ {radical}{ital s} ) from data on a single-spin production asymmetry such as {ital A}{sub {ital N}d}{sigma}({ital pp}{sub {up arrow}}{r arrow}{pi}{sup 0}{ital X}) at large transverse momentum can be used to argue for an underlying hard-scattering'' mechanism. Data from the upcoming Fermilab polarized beam experiment (E-704) can be used to test the scaling hypothesis.

Theoretical analyses of polarized leptoproduction data suggest that the polarized gluon structure function might be large, but there has been no independent measurement of this quantity. Measurements of single spin asymmetries in the production of muon pairs from the scattering of two protons, one of which is longitudinally polarized, can be interpreted in terms of polarized gluon and polarized quark structure functions. Here we compute the asymmetries for the parton subprocesses that contribute to the measured muon pair production.