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We show how the helicity and angular dependence of large-momentum-transfer exclusive processes can be used to test the gluon spin and other basic elements of perturbative quantum chromodynamics (QCD). Unlike inclusive reactions, these processes isolate QCD hard-scattering subprocesses in situations where the helicities of all the interacting quarks are controlled. The predictions can be summarized in terms of a general spin selection rule which states that the total hadron helicity is conserved (initial^₇=final^₇) up to corrections falling as an inverse power in the momentum transfer. In particular, the hadrons in e^+e^-^*h₀+h₁ are produced at large Q^2 with opposite helicity ₀+₁=0, and |₈|12. This also implies ddcos (1+cos^2) for all baryon pairs and ddcossin^2 for all meson pairs, to leading order in 1Q. Applications to many processes are given, including electroweak form factors, two-photon processes, hadron-hadron scattering, and heavy-quark decays (e. g. , ).
Brodsky et al. (Tue,) studied this question.