Abstract
Quantum chromodynamics (QCD) is the theory of strong interactions of quarks and gluons collectively called partons, the basic constituents of all nuclear matter. Its non-abelian character manifests in nature in the form of two remarkable properties: color confinement and asymptotic freedom. At high energies, perturbation theory can result in the growth and dominance of very gluon densities at small-x. If left uncontrolled, this growth can result in gluons eternally growing violating a number of mathematical bounds. The resolution to this problem lies by balancing gluon emissions by recombinating gluons at high energies: phenomena of gluon saturation. High energy nuclear and particle physics experiments have spent the past decades quantifying the structure of protons and nuclei in terms of their fundamental constituents confirming predicted extraordinary behavior of matter at extreme density and pressure conditions. In the process they have also measured seemingly unexpected phenomena. We will give a state of the art review of the underlying theoretical and experimental tools and measurements pertinent to gluon saturation physics. We will argue for the need of high energy electron-proton/ion colliders such as the proposed EIC (USA) and LHeC (Europe) to consolidate our knowledge of QCD knowledge in the small x kinematic domains.
Highlights
Quantum chromodynamics (QCD) is the theory of strong interactions of quarks and gluons collectively called partons, the basic constituents of all nuclear matter
For a hadron moving in the plus light-cone direction with large momentum P+ probed at the scale x0P+, with x0 1, the Color Glass Condensate (CGC) separates the partonic content of hadrons according to their longitudinal momentum k+ = xP+, where x refers to the longitudinal momentum fraction of the parton probed in the nucleus/nucleon
In this document we have presented various observables that may pave the road for the discovery of gluon saturation at existing and future collider experiments
Summary
Quantum chromodynamics (QCD) is the theory of strong interactions of quarks and gluons collectively called partons, the basic constituents of all nuclear matter. Asymptotic freedom, on the other hand, states that at short distances, quarks and gluons interact weakly due to smallness of the coupling constant αs at asymptotically high energies The latter property formed the theoretical basis behind the parton model [4,5,6], allowing the use of perturbation theory while leading to the successful description of a plethora of experimental results from fixed target experiments to high energy colliders. High-energy nuclear and particle physics experiments have spent the past decades quantifying the structure of protons and nuclei in terms of their fundamental constituents confirming extraordinary behaviour of matter at extreme density and pressure conditions In the process they have measured seemingly unexpected phenomena which will need a new generation of theoretical efforts as well as pertinent collider experiments.
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