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 {ital Q}{sup 2} is increased over the region 1 GeV{sup 2}{lt}{ital Q}{sup 2}{lt}100 GeV{sup 2}. In the asymptotic region {ital Q}{sup 2}{gt}1000 GeV{sup 2}, a saddle-point approximation yields suppression by a fraction of a power of {ital Q}{sup 2}. {open_quote}{open_quote}Chirally odd{close_quote}{close_quote} exclusive wave functions which carry a nonzero orbital angular momentum, and yet are leading order in the high energy limit, play an important role. {copyright} {ital 1996 The American Physical Society.}