Hadron helicity violation in exclusive processes: Quantitative calculations in leading order QCD

We study a new mechanism for hadronic helicity flip in high energy hard exclusive reactions. The mechanism proceeds in the limit of perfect chiral symmetry, namely, without any need to flip a quark helicity. The fundamental feature of the new mechanism is the breaking of rotational symmetry of the hard collision by a scattering plane in processes involving independent quark scattering. We show that in the impulse approximation there is no evidence for strong suppression of the helicity-violating process as the energy or momentum transfer $Q^2$ is increased over the region $1 \mathrm{Ge}{\mathrm{V}}^2<Q^2<100\mathrm{} \mathrm{Ge}{\mathrm{V}}^2$. In the asymptotic region $Q^2>1000\mathrm{}$ Ge${\mathrm{V}}^2$, a saddle-point approximation yields suppression by a fraction of a power of $Q^2$. "Chirally odd" exclusive wave functions which carry a nonzero orbital angular momentum, and yet are leading order in the high energy limit, play an important role.