Helicity selection rules and tests of gluon spin in exclusive quantum-chromodynamic processes

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 (Σinitial^λH=Σfinal^λH) up to corrections falling as an inverse power in the momentum transfer. In particular, the hadrons in e⁺e^-→γ*→hA+h̄B are produced at large Q² with opposite helicity λA+λB=0, and |λᵢ|≤1/2. This also implies dσdcosθ∝(1+cos²θ) for all baryon pairs and dσdcosθ∝sin²θ for all meson pairs, to leading order in 1/Q. Applications to many processes are given, including electroweak form factors, two-photon processes, hadron-hadron scattering, and heavy-quark decays (e.g., ψ→pp̄).