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 Q² is increased over the region 1 GeV²<Q²<100 GeV². In the asymptotic region Q²>1000 GeV², a saddle-point approximation yields suppression by a fraction of a power of Q². "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.