Quantum Chromodynamics at small Bjorken-x
3 July 2017
UAntwerpen, Stadscampus, Willem Elschotzaal (Hof Van Liere) - Prinsstraat 13 - 2000 Antwerpen (route: UAntwerpen, Stadscampus
Organization / co-organization:
Department of Physics
Pierre Van Mechelen
PhD defence Pieter Taels - Faculty of Science, Department of Physics
In particle accelerators such as the Large Hadron Collider (LHC) in CERN, Geneva, protons are collided with a very high energy. In such a collision, the proton breaks up and reveals its inner structure: a complicated system of quarks and gluons —elementary building blocks of matter, interacting with each other through the strong interaction.
The strong interaction between quarks and gluons is described by a theory known as Quantum Chromodynamics (QCD). Although the construction of this theory was already finished in the mid-seventies, and its correctness was confirmed through the successful description of many experiments, the question of how the interaction of quarks and gluons leads to protons is still an open problem. Since protons are the constituents of the nucleus, which is the center of an atom, the LHC can in fact be seen as a gigantic microscope which probes the ‘microscopic’ structure (quarks and gluons) of ‘macroscopic’ objects (protons).
In particular, nowadays —with the advent of more powerful particle accelerators— it is possible to investigate the structure of protons in a regime in which those gluons play a role that merely carry a very small fraction (Bjorken-x) of their proton’s energy. In this regime, the gluon density can become so large that nonlinear effects start to play a role. Theoretically, this situation is described by the so-called Color Glass Condensate (CGC), a theory within QCD especially developed for this problem.
The first part of this thesis is an overview of QCD at high energies, followed by an introduction to the CGC. The rest of the work is devoted to two different projects, in which each time the CGC is applied to a problem within QCD.
In the first project, we study the three dimensional gluon densities (gluon TMDs) of a nucleus from the point of view of the CGC. In contrast to one dimensional gluon distributions, which are well-known both theoretically and experimentally, gluon TMDs are dependent on the specific process under consideration. We elucidate on this problem by bringing in information from the CGC, and we use this theory to model the different densities.
In the second project, we investigate the problem of gluon radiation inside a nuclear medium. This is relevant for the experiments conducted at the LHC or in RHIC (Brookhaven, USA) in which, instead of protons, heavy lead or gold ions are collided. In such collisions, the so-called Quark-Gluon Plasma (QGP) is formed, in which, during a fraction of a second, quarks and gluons interact freely in a regime of an incredibly high density and temperature. The study of how radiation works inside such a medium, which is reminiscent of our universe in the first moments after the Big Bang, allows for a better investigation of the QGP.