Separating coherent and incoherent photoproduction at an Electron-Ion Collider: Comparing lead, gold and silver beams

Abstract: Exclusive photoproduction is an important probe of nuclear parton distributions at low Bjorken-$x$. The Good-Walker paradigm relates coherent photoproduction to the average nuclear configuration, giving access to the transverse distribution of gluons in a target, while incoherent photoproduction is sensitive to fluctuations such as gluonic hotspots. Efficiently separating coherent and incoherent interactions is a key challenge for experiments at the future Electron-Ion Collider (EIC). In this article, we study nuclear de-excitation via photon emission, and see how the finite photon detection efficiency leads to misidentification of incoherent emission. We compare the detection efficiency for incoherent $J/ψ$ production using the BeAGLE event generator for Ag-107, Au-197, and Pb-208 targets, for two different forward photon energy thresholds: 50 MeV and 200 MeV. The shell structures of the three species exhibit different incoherent tagging efficiencies due to differences in the lifetimes and energy levels of their excited states. Some of the low-lying states live long enough so that they decay outside the EIC detectors. Especially for the lower photon energy threshold, lead allows a higher tagging efficiency than gold, due to its lack of low-lying and/or long-lived excited states.