Eur. Phys. J. C (2022) 82: 837
https://doi.org/10.1140/epjc/s10052-022-10774-3

Regular Article - Theoretical Physics

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

Department of physics, Indian Institute of Technology Delhi, New Delhi, India

^a arjun.kumar@physics.iitd.ac.in

Received: 16 June 2022
Accepted: 31 August 2022
Published online: 26 September 2022

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.