(protons or nuclei) is larger than their diameter. The protons only interact through the electromagnetic force [@bauretal], enabling the $\gamma p$ or $\gamma \gamma$ subprocesses, where the photons are described by a Weizsäcker-Williams distribution for each proton. Thus, in Peripheral Collisions the photons are treated as quasi-real particles due to the softness of its momentum.

The Fig. \[fig:foto-part\] shows the Feynman diagram for the $\gamma^{*}q$ subprocess, which represents only one contribution for the process, other possibilities are obtained exchanging the fermion lines in the color dipole. Moreover, the central exclusive production by the gluon-annihilation vertex yields four distinct diagrams, all of them needed to fully account for the process. In Ref. [@bialas] it is shown that the sum of the four possible diagrams results in the discontinuity of the diagram shown in Fig. \[fig:foto-part\].

The two-upper blobs represent the effective vertices of the photon-gluon coupling which can be obtained through the impact factor formalism. The same formalism is used to explore the process with a non-zero momentum transfer with two gluons exchanged in the $t$-channel [@forshaw-evanson]. The other blob represents the gluon distribution function in the proton.

The process that we study in this paper is based on the partonic 