GPDs and DVCS with Positrons

The beam charge asymmetry helps to isolate the real part of the deeply virtual Compton scattering (DVCS) amplitude. It is discussed what information can be gained both from the real and imaginary part of the DVCS amplitude. INTRODUCTION Form factors are the coherent amplitude that the nucleon remains intact when one of its quarks absorbs a certain momentum transfer ∆μ . In this amplitude, the contribution from all quarks carrying all kinds of momenta is added up coherently. Compton scattering provides a more surgical approach (Fig. 1). Due to the presence of the quark propagator between the two photon vertices (note that for virtual photons with large virtuality, the Compton amplitude is dominated by handbag diagrams), the Compton amplitude is sensitive to the momentum fraction x carried by the active quark. Suppose one could disect the Dirac form factor F 1 or the Pauli form factor F q 2 for quarks with flavor q w.r.t. the (average) momentum fraction x = 1 2 (