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 of the helicity violating process as the energy or momentum transfer Q2 is increased over the region 1GeV 2 < Q2 < 100GeV 2. In the asymptotic region Q2 > 1000 GeV2, a saddle point approximation with doubly logarithmic accuracy yields suppression by a fraction of power of Q2. “Chirally–odd” exclusive wave functions which carry non–zero orbital angular momentum and yet are leading order in the high energy limit, play an important role.