Narrow Results

We review the color glass condensate effective theory, that describes the gluon content of a high energy hadron or nucleus, in the saturation regime. The emphasis is put on applications to high energy heavy ion collisions. After describing initial state factorization, we discuss the glasma phase, that precedes the formation of an equilibrated quark–gluon plasma. We end this review with a presentation of recent developments in the study of the isotropization and thermalization of the quark–gluon plasma.

Motivated by recent discussions of entanglement in the context of high energy scattering, we consider the relation between the entanglement entropy of a highly excited state of a quantum system and the classical entanglement entropy of the corresponding classical system. We show on the example of two weakly coupled harmonic oscillators that the two entropies are equal. Quantum mechanically, the reduced density matrix which yields this entropy is close to the maximally entangled state. We thus observe that the nature of entanglement in this type of state is purely classical.

In this paper, we review recent progress in the description of unpolarized transverse-momentum-dependent (TMD) gluon distributions at small $x$ in the color glass condensate (CGC) effective theory. We discuss the origin of the nonuniversality of TMD gluon distributions in the TMD factorization framework and in the CGC theory and the equivalence of the two approaches in their overlapping domain of validity. We show some applications of this equivalence, including recent results on the behavior of TMD gluon distributions at small $x$, and on the study of gluon saturation. We discuss recent advances in the unification of the TMD evolution and the nonlinear small-$x$ evolution of gluon distributions.

Ultra-high energy neutrinos are an enigma; among their many poorly understood aspects are their origins and how they interact with nucleons when they reach the Earth. Due to the hard scale ($Q\sim M_{W,Z}$) involved in neutrino-nucleon scattering and for a large range of neutrino energies, it is appropriate to describe the target nucleon in terms of its partons — quarks and gluons — and their evolution with $Q^2$ as governed by the Dokshitzer–Gribov–Lipatov–Altarelli–Parisi (DGLAP) evolution equations of perturbative Quantum ChromoDynamics (pQCD). Nevertheless, at the highest neutrino energies, the scattering cross-section is dominated by the contribution of small $x$ gluons of the target where one expects DGLAP evolution equations to break down due to high gluon density effects (gluon saturation). Here, we give a brief overview of gluon saturation physics in QCD and its effects on ultra-high energy neutrino-nucleon (nucleus) scattering cross-section.

The high-energy factorization and the associated B-JIMWLK or BK evolution equations are presented, using the example of DIS structure functions. The necessity of taking gluon saturation into account is discussed, and also the various approximations underlying high-energy factorization. The appearance of large NLL corrections in such a framework with or without gluon saturation is recalled, and their physical origin is explained. Finally, old and new results are presented about the resummation of some of those large corrections, related to kinematical approximations.

In this talk, I demonstrate that the proposed Electron-Ion Collider (EIC) will be an ideal and unique future facility to address many overarching questions about QCD and strong interaction physics at one place. The EIC will be the world's first polarized electron-proton (and light ion), as well as the first electron-nucleus collider at flexible collision energies. With its high luminosity and beam polarization, the EIC distinguishes itself from HERA and the other fixed target electron-hadron facilities around the world. The EIC is capable of taking us to the next QCD frontier to explore the glue that binds us all.

We summarize the results of a recent first computation in the Color Glass Condensate effective field theory (CGC EFT) of the next-to-leading order (NLO) impact factor for inclusive photon+dijet production in electron-nucleus (e+A) deeply inelastic scattering (DIS) at small x. Our computation simultaneously provides the ingredients to compute fully inclusive DIS, inclusive photon, inclusive dijet and inclusive photon+jet channels to the same accuracy. We outline the Wilsonian renormalization group (RG) procedure by virtue of which extant results for the next-to-leading log (in x) JIMWLK evolution, can be integrated with our results to improve the precision to $O\left({\alpha }_s^3\ln \left(1/x\right)\right)$ accuracy. This paves the way towards quantitative studies of saturation at a future Electron-Ion Collider.

This proceeding article is aimed at drawing attention to selected theoretical issues on quarkonium production models and its factorization in proton-proton and proton-ion nucleus collisions within the color-glass-condensate framework.