This paper looks at a practical problem for the planned U.S. Electron‑Ion Collider (EIC): how well experiments can tell two kinds of photon‑induced reactions apart. “Coherent” photoproduction probes the average distribution of gluons in a whole nucleus. “Incoherent” photoproduction happens when the nucleus is left in an excited state, and it probes event‑by‑event fluctuations such as small dense spots of gluons. Experiments separate the two by looking for decay products from the excited nucleus, especially low‑energy photons emitted when the nucleus relaxes to its ground state.

The authors used the BeAGLE event generator to simulate incoherent J/ψ photoproduction on three nuclei: silver‑107, gold‑197 and lead‑208. BeAGLE combines several simulation packages (PYTHIA‑6 for the initial interaction, DPMJET for the intranuclear cascade, and FLUKA for nuclear evaporation and gamma de‑excitation). They tested how the probability to spot the nuclear de‑excitation photons changes when forward photon detectors only see photons above a chosen energy cutoff. Two detector thresholds were compared: 50 MeV and 200 MeV.

The key physics comes from nuclear shell structure. The lowest excited states and their lifetimes differ strongly between the three species. The first excited‑state energies are about 77 keV for Ag‑107, 93 keV for Au‑197, and 2.6 MeV for Pb‑208. Lead is “doubly magic,” so its first excited state is much higher in energy. That higher energy reduces the number of incoherent reactions that only transfer a tiny amount of energy, and when lead does emit de‑excitation photons they tend to be more energetic and easier to detect. By contrast, silver and gold have low‑lying states that can live long enough that the photons are emitted after the ion has left the detector region, or the photons are too low in energy to pass realistic detector thresholds. For the lower 50 MeV threshold, the study finds that lead offers a higher tagging efficiency than gold for detecting incoherent breakup.