Investigating the structure of gluon fluctuations in the proton with incoherent diffraction at HERA

Abstract

Impact parameter dependent dipole models are ideal tools for investigating the spatial structure of the proton. We investigate the incoherent ep cross section in exclusive J/psi photoproduction as measured by HERA, and find that as |t| increases, the models need several levels of substructure of gluon density fluctuations in order to describe the measured data well. In lieu of a perturbative description, we add this substructure by hand. This substructure is modelled as hotspots within hotspots. This enables us to describe measurements for |t|> 1 GeV^2, which is necessary for describing any observable which integrate over the t-spectrum, such as the rapidity or W_{gamma p}. We find that three levels of proton substructure are adequate for a good description of all available ep data up to |t|=30 GeV^2. We note that the gluonic density fluctuation structure follows a scaling behaviour, such that the logarithms of the number of hotspots and their size fall on a line, effectively reducing the available parameter space of the model. Our findings systematically constrains and provides a benchmark for the development of a perturbative model of spatial gluon fluctuations in nucleons.