HELICITY ASYMMETRY FOR PROTON EMISSION FROM POLARIZED ELECTRONS IN THE EIKONAL REGIME

The nuclear response to longitudinally polarized electrons, detected i n coincidence with out-of-plane high-energy protons, is discussed in a simple model where the ejectile wave function is approximated as a pl ane wave with a complex wave vector. This choice is equivalent to solv ing the problem of final-state interactions (FSI) in homogeneous nucle ar matter, as the residual nucleus can be described to a first approxi mation when dealing with very fast emitted protons. The main advantage of the present method is that in the framework of the distorted-wave impulse approximation at the one-photon exchange level it allows for a n analytical derivation of all the components of the nuclear response, including the so-called fifth structure function f(01)', which is ver y sensitive to FSI. The imaginary part of the complex wave vector prod uces purely geometrical FSI effects and, consequently, breaks the symm etry of the cross section with respect to the incoming electron helici ty. Inspection of every single contribution in the analytical formulas , here considered up to the fourth order in the nonrelativistic reduct ion in powers of the inverse nucleon mass, allows for a detailed study of the role of each elementary reaction mechanism. In particular, can cellations among the leading contributions determine the very small ab solute size of f(01)' and produce a nontrivial asymptotic scaling of t he related helicity asymmetry for large values of the momentum transfe r.