Imaging the nucleus with high-energy photons

Understanding how quarks and gluons behave in the nuclear environment is an important focus of modern nuclear physics. Recent measurements have provided an improved understanding of how quark and gluon densities are altered in heavy nuclei. It has also become possible to make multi-dimensional pictures of the nucleus, exploring how these alterations are distributed within heavy nuclei. The modifications are naturally expected to be largest in the core of a nucleus and smaller near its periphery; this variation can change the effective shape of the nucleus. Experiments have also started to explore the transverse momentum distribution of the partons in the nuclei, and, by using incoherent photoproduction as a probe, measure event-by-event fluctuations in nucleon and nuclei parton densities. This Review explores recent progress in measurements of nuclear structure at high energy, emphasizing these multi-dimensional pictures. We also discuss how a future electron-ion collider with high luminosity and centre-of-mass energy will make exquisitely detailed images of partons in a nucleus. The partonic (quark and gluon) structure of protons and neutrons is modified in heavy nuclei. This Review surveys how studies of photon-induced interactions reveal the density distribution of partons in nuclei, thereby probing quantum chromodynamics in high-density environments. Key pointsThe partonic (quark and gluon) content of the nucleus is different from what one would get by simply combining protons and neutrons.The density of partons carrying only a small fraction of the nucleon momentum (partons with small Bjorken-) is very high. The resulting increase in parton-parton interactions modifies the parton momentum distribution and may lead to qualitatively different behaviour.xThe high-energy photons that are produced in ultra-peripheral collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) can be used to study nuclear structure at small Bjorken-.xCoherent vector meson photoproduction can be used to determine the location of interactions within the nucleus, thereby probing the spatial dependence of nuclear modifications to the parton distribution.Incoherent vector meson photoproduction can be used to study event-by-event fluctuations in nuclear parton distributions and thereby determine whether nuclei are smooth or clumpy.Studies of photoproduction and electroproduction at a future electron-ion collider should make it possible to make detailed three-dimensional maps of parton distributions.