Determine whether saturation or shadowing better represents the underlying nuclear QCD dynamics

Determine which of the two theoretical frameworks proposed for high-energy photon–nucleus interactions—gluon saturation models that describe non-linear dynamics of gluon densities, and nuclear shadowing models that account for multiple scatterings of the probe in the nuclear color field (leading-twist shadowing)—better represents the underlying QCD physics responsible for the observed reduction of gluon density in nuclei compared to A independent nucleons.

Background

In photon–nucleus collisions at high energies, experiments have established that the gluon density in a nucleus composed of A nucleons is reduced relative to the expectation from A free nucleons. Two principal frameworks have been developed to explain this: saturation models, which invoke non-linear gluon dynamics leading to a balance between splitting and recombination, and shadowing models, which describe multiple scatterings of the probe within the nuclear color field.

To date, both saturation and shadowing approaches have successfully described various observables, including coherent and incoherent photoproduction measurements. This success has left an unresolved question regarding which framework more accurately captures the underlying physics of nuclear gluonic structure in these processes.