Double parton scattering via photon-proton interactions: a new light on the transverse proton structure

In this contribution we present the main results of the investigation about double parton scattering (DPS) in quasi-real photon proton interactions. We show the first evaluation of the DPS cross-section at leading-order for the four-jet photo-production observed at HERA. To this aim the $\gamma-p$ effective cross section has been computed for the first time. One of the main outcomes of this analysis is that the DPS contribution is not negligible and potentially measurable. Furthermore, possible future data could be used to get new information on the transverse proton structure not accessible in other processes.