subprocess $\gamma^{*} q \to \gamma^{*} + H + q$ shown in Fig. \[fig:foto-h\]. The central line cuts the diagram and expresses the use of the Cutkosky rules in order to obtain the imaginary part of the scattering amplitude, which is given by $$\begin{aligned}
{\rm{Im}}A = \frac{1}{2} \int d(PS)_{3} \, {\cal{A}}_{L} \, {\cal{A}}_{R}
\label{cut-rule}\end{aligned}$$ with ${\cal{A}}_{L}$ and ${\cal{A}}_{R}$ being the amplitudes on the left- and right-hand side of the cut, respectively, and $d(PS)_{3}$ is the volume element of the three-body phase space. The scattering amplitude of the process is treated essentially as an imaginary quantity in view of the vacuum quantum numbers of the exchanged particle [@foldy]. Moreover, the fermion loop is divided in two distinct parts, each other representing the splitting of the photon. In the Dipole Model [@mueller1] the splitting of the photon into a quark-antiquark pair requires an wave function, where its product with the complex conjugate represents the fermion loop. In the photoproduction approach, the impact factor formalism is used to describe the color dipole.

The representation of the partonic subprocess by the diagram in Fig. \[fig:foto-h\] shows a single process of $\gamma^{*}q$ interaction. The other diagrams can be obtained exchanging the coupling of the gluon lines 