Evaluation of the Nucleon Helicity Flip Form Factor using One and Two   Virtual Photons
  In this work, we will evaluate the nucleon helicity flip form factor at the limit of large momentum transfer. Hereby, we will study the exchange of one and two virtual photons separately. For the calculation of the scattering amplitudes and nucleon transition probability matrix elements, we combine QCD perturbation theory with an expansion in nucleon distribution amplitudes. Using the combination of leading and sub-leading twist nucleon distribution amplitudes, one obtains the desired form factor. Using this technique, we will obtain a divergent result for the form factor. Nevertheless, the structure of the divergency can be extracted. Finally, we will comment the obtained expressions and discuss the behavior in unpolarized and polarized cross sections. 
Thorsten Sachs and Patrick Sturm
hep-ph