Quantum Chromodynamics at small Bjorken-x

Abstract

With the advent of very powerful particle accelerators, such as RHIC and the LHC, it becomes possible to study QCD in high energy collisions, in which the gluon content of the proton or nucleus is probed and its density becomes often large enough for nonlinear effects to play a role. This small-x regime of QCD is well described by an effective theory known as the Color Glass Condensate (CGC). In this thesis, we introduce the CGC and apply it to two different problems. First, we use the CGC to study forward heavy-quark production in pA collisions. When the quarks are nearly back-to-back, the CGC result coincides with the one in the TMD factorization approach. This allows us to extract the small-x limit of the Weizsäcker-Williams gluon distribution, as well as the dipole distribution and one extra gluon TMD. Each of these gluon TMDs is accompanied by a partner, which couples via the quark mass and which describes the linearly polarized gluon content of the unpolarized nucleus. We calculate the six resulting gluon TMDs analytically in the MV model, and evolve them in rapidity using a numerical implementation of JIMWLK. The second problem is situated within heavy-ion physics. Jets, produced in the scattering of two nuclei, travel through the Quark-Gluon Plasma (QGP) before reaching the detector, and are attenuated as a result of their interaction with this medium. This phenomenon, known as jet quenching, is one of the main probes to investigate the QGP. We focus on the transverse momentum broadening of a hard particle traveling through a nuclear medium, and employ small-x techniques to attempt to resum the leading logarithmic corrections due to soft gluon radiation. Although, ultimately, we can only solve the resulting in-medium evolution equation to DLA accuracy, we do present a concise framework for the problem, and draw a detailed comparison with the CGC and with the literature.