Fig. 1
From: A glimpse of gluons through deeply virtual compton scattering on the proton
A few examples of DVCS diagrams. At leading-order in perturbative quantum chromodynamics (QCD) (a), the virtual photon with four-momentum q interacts with a single quark (single straight line) from the proton p, in the limit Q 2 = −q 2 much larger than the proton mass squared. Subsequently, the active quark emits a real photon with four-momentum q′. The recoil proton has four-momentum p′. Perturbation theory can be used to calculate the part of the amplitude above the (dashed) factorization line, whereas GPDs encode the non-perturbative structure of the nucleon. At next-to-leading order in perturbative QCD (b), a gluon (curly line) from the proton splits into a quark-antiquark pair and the quark absorbs the virtual photon. c An example of deeply virtual Compton scattering (DVCS) diagram at next-to-leading twist illustrating a quark-gluon correlation. The average longitudinal momentum fraction carried by the active parton (quark/gluon) is x and −2ξ is the longitudinal momentum transfer. The helicity of the photons contributing to the leading-twist amplitudes are specified in parenthesis
