The relativistic treatment of the quasifree nucleon-nucleus scattering has
been described in [SRM86], [HoI86], [HoM88], [HiD94]
and in
particular for charge exchange reactions in [HoP93]. The method
generally employed uses a relativistic random phase approximation to the
Walecka model. In the Walecka model the nn interaction is mediated by the
exchange of isoscalar scalar 
and vector 
mesons, and
in the mean-field approximation these meson fields are replaced by their
classical expectation value.
In the mean-field approximation strong scalar
and vector mean-fields will cause large shifts in the in-medium mass and
energy of nucleons in the nucleons:
where 
is the mean scalar field and 
is the
free(in-medium) nucleon mass.
The modified mass of the nucleon is in
itself a reasonable explanation for the lack of enhancement because the
reduced value of the nucleon mass should have a significant effect on
the longitudinal spin-isospin response. An RPA calculation of the pion
dimesic function [DaP91] gives the effective 
coupling constant
in the nuclear medium as [HoP93]
If the pion coupling constant is reduced in-medium one would expect that pionic correlations will play a diminished role in an RPA calculation of the nuclear responses. Therefore, the spin-longitudinal response, which was primarily mediated by pion exchange, will be smaller relative to the nonrelativistic case.
There is also another effect due to the change in effective nucleon mass.
The peak of
the quasifree distribution, 
, will be
shifted relative to a naive Fermi gas model, and the distribution will
broaden, 
before any
correlations
are considered. Therefore, attractive pion correlations will act on an
uncorrelated case which is already ``hardened'' (moved to higher energy)
and the overall effect of the pion correlations much smaller than without
the relativistic corrections.