PWBA: Plane Wave Born Approximation
DWBA: Distorted Wave Born Approximation
In PWBA, the incident wave is approximated as plane wave. But in DWBA, the incident wave is approximated as distorted wave.
The direct reaction theories are based upon the distorted wave born approximation (DWBA) which gives more accurate description of direct reactions such as inclastic scattering, stripping reactions, pickup reactions etc. By PWBA, backward peaking can be understood.
The factors which are ignored in DWBA are:-
(i) The incident wave is no longer approximated as plane wave, rather it is considered to be as distorted by nuclear interaction.
(ii) The potentials are assumed to be independent of momenta. That is, the Born approximation ignores the strong interaction between the nucleus and incident nucleon.