Phase transitions in holographic QCD and instanton crystals

Abstract

We investigate phase transitions in holographic models of QCD. In chapter I, we explore the effect of constant external U(1) fields on the physics of chiral symmetry breaking, as realized in the D3/D7 model. We discover that this model exhibits the phenomenon of magnetic catalysis, which is what one would expect from a weakly coupled field theory intuition. In chapter II, we continue exploring the effect of external U(1) fields but now on the backreacted D3/D7 model, where the backreaction is obtained via a smearing procedure. We again find the magnetic catalysis effect, however the results differ from the previous case depending on the backreaction parameters. In chapter III, we investigate lattices of instantons in the D4/D8 model of chiral symmetry breaking. These instanton lattices can change dimensionality, and in particular we investigate the 1D [right arrow] 2D transition as a simpler case of the more complicated 3D [right arrow] 4D transition which is conjectured to be holographically dual to the baryonic to quarkyonic phase transition. Besides this interpretation, one could also view this as a hypothetical condensed matter system. We have a lattice of instantons dominated by two-body forces, whose interactions depend not only on their mutual distance in physical space but also on their relative orientations in the internal isospace. We obtain a rich variety of instanton crystals whose description could serve to be useful beyond holography.