Shining Light on Saturated Gluons

In this project the properties of hadronic matter with emergent non-linear saturation phenomena at extremely large parton densities are theoretically determined. This is achieved by studying Quantum Chromodynamics (QCD), describing the strong interactions between quarks and gluons, at high energies.There are solid theoretical arguments formulated in the Color Glass Condensate (CGC) effective field theory to suggest that saturation effects play a major role in hadronic interactions at high energies. However, so far no clear signal of gluon saturation have been observed. This will change when the Electron-Ion Collider (EIC) starts to measure photon-mediated electron-nucleus collisions allowing for precision studies of high density hadronic matter in heavy nuclei.We develop the CGC approach to high-energy QCD to the level where, for the first time, multiple scattering processes can be simultaneously described from the unified framework at next-to-leading order accuracy. These developments are necessary to bring the theoretical framework to the level required to probe non-linear dynamics in the EIC era.We extract the non-perturbative proton structure from global analyses and determine if non-linear QCD dynamics is observable in current collider energies. We calculate predictions for the EIC and determine how different processes at the EIC probe gluon saturation. Before the EIC, photon-induced processes at high energies are available at the LHC in ultra peripheral collisions that we use to extract, for the first time, the effect of non-linear dynamics on the nuclear high-energy structure at NLO accuracy. The fundamentally important results are applied to develop a new description for the initial condition of heavy ion collisions where Quark Gluon Plasma (QGP) is produced. We determine the effect of NLO initial state description on the extraction of fundamental QGP properties, and quantify the synergies between the LHC heavy ion program and future EIC.