Mining for Gluon Saturation at Colliders

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.

Dijet impact factor in DIS at next-to-leading order in the Color Glass Condensate

“…In this saturation regime, gluons attain large occupation numbers ∼ 1/α s , for which the appropriate description is in terms of strong classical fields [7][8][9]. The Color Glass Condensate (CGC) is an effective field theory (EFT) describing the properties of these overoccupied small-x gluons and it has been employed to study numerous observables in electron-nucleus, proton-nucleus and nucleus-nucleus collisions [10][11][12][13][14][15].…”

Accessing subnuclear fluctuations and saturation with multiplicity dependent $J/ψ$ production in p+p and p+Pb collisions

“…Observing signals of gluon saturation in hadronic collisions as well as deeply inelastic scattering has been a long standing goal in the high energy nuclear physics community. While many observables are compatible with saturation model results or even more sophisticated color glass condensate (CGC) effective field theory calculations, see [1][2][3] and references therein, so far there has been no clear evidence that we have reached the kinematic regime where gluon saturation is present and strongly affects the particle production [4][5][6].…”

“…2. The variable k ⊥ is the transverse momentum transfer to the quark in the conjugate amplitude 6 . The color correlator Ξ κ depends on the color state of the cc, octet [8] or singlet [1], which at leading order in N c are given by Ξ [8]…”

Deep Inelastic Scattering in the Dipole Picture at Next-to-Leading Order

“…This of course requires theory calculations of the scattering processes in the CGC formalism, which are plentiful -see Refs. [15][16][17][18][19][20] for reviews of phenomenological studies of different observables.…”

“…This phenomenon is known as saturation, and in this work it is studied in the Color Glass Condensate (CGC) effective field theory, which describes QCD at very high energies. The CGC formalism has been used to describe strong interactions in a large variety of scattering processes in e+p, e+A, p+A, and A+A collisions [15][16][17][18][19][20]. Further, it has the power to ab initio describe thermalization in heavy ion collisions, and the initial conditions of thermalized quark gluon plasma.…”