HOLOGRAPHIC DESCRIPTIONS OF QCD UNIVERSITY OF SOUTHAMPTON FACULTY OF SCIENCE PHYSICS by Andrew John Mark Tedder Abstract Doctor of Philosophy The AdS/CFT correspondence has long been used as a tool for understanding non-perturbative phenomena in gauge theories because it is an example of a ‘strong-weak’ duality: when one side of the duality is weakly coupled, the other is strongly coupled and vice-versa. Hence strongly coupled phenomena can be studied by looking at the weakly coupled side of the duality. In its original form the correspondence proposes a duality between type IIB superstring theory on AdS$_{5}$ × S 5 and an N = 4 supersymmetric Yang-Mills theory in four dimensions. In this thesis we investigate proposed duals to QCD itself. Duals to QCD fall into two categories: ‘top-down’ and ‘bottom-up’. We take inspiration from both by truncating a consistent solution to the type IIB supergravity equations of motion (top-down). This model demonstrates dynamical chiral symmetry breaking, has a running coupling and contains a holographic description of the vector meson sector. By artificially extending the existing U(1) symmetry to SU(2) (bottom- up) we then obtain a holographic description of the axial vector sector. We show that this model reproduces the masses and decay constants of the lightest mesons to the 10% level. By regulating the UV with a sharp cut-off we can reproduce the ρ meson masses to within 2%. Finally we demonstrate that this model can be used to reproduce a very good agreement with hadronization data for particle production over a range of four orders of magnitude. i