Holographic Bilayer/Monolayer Phase Transitions

In the present work, we discuss the phase structure of bilayer and monolayer phases in the (2+1)-dimensional defect field theory whose gravity dual is obtained by embedding D5/anti-D5 probe flavour branes in the singular conifold. We study in detail the embedding equations and compare the free energies of the resulting configurations at non-zero temperature and external magnetic field perpendicular to the defects. Moreover, we analyse the meson spectrum and confirm the stability of the single bilayer solution.Comment: 24 pages, 8 figure

A Novel (2+1)-Dimensional Model of Chiral Symmetry Breaking

We propose a new model of flavour chiral symmetry breaking in a (2+1)-dimensional defect gauge theory of strongly coupled fermions by introducing probe D5/anti-D5-flavour branes on the conifold. After working out the flavour brane embeddings at zero temperature, we thoroughly investigate the spectra of small fluctuations on the world volume of the flavour branes (meson spectrum) and conclude that they are free of tachyons. Thus the proposed probe brane embedding is stable. Moreover, we introduce finite temperature and an external magnetic field and study the thermodynamics of the resulting configurations. Namely, we compute the free energies, entropies, heat capacities and magnetisations. The results are used to establish a detailed phase diagram of the model. We find that the effect of magnetic catalysis of chiral symmetry breaking is realised in our model and show that the meson-melting phase transition coincides with the chiral symmetry breaking phase transition. Furthermore, we show that the model is in a diamagnetic phase.Comment: 1+28 pages, 23 figures, PDFTeX, v2: comments and refs. added, version accepted for publication by JHE

Topics in flux compactifications of type IIA superstring theory

textRealistic four-dimensional model building from string theory has been a focus of the string theory community ever since its inception. Toroidal orientifold constructions have emerged as a technically simple class of candidate models. Novel ingredients, such as background fluxes, have been discovered and intensely studied over the past few years. They allow for a (partial) solution of several long standing problems associated with model building in this framework. In this thesis, I summarize progress that has been made in toroidal orientifold constructions in type IIA string theory.This includes a detailed discussion of moduli stabilization and (non-) supersymmetric AdS and Minkowski vacua. Furthermore I commence a systematic study of generalized NSNS, i.e., metric and non-geometric, fluxes. The emergence of novel D-terms is presented in detail. While most of the discussion applies to generic orientifolds of T⁶, most features are exemplified by and studied in terms of a certain orientifold of T⁶/ℤ₄ owing to its somewhat richer structure compared to simpler models studied before. It is also briefly reported on efforts of finding de Sitter vacua and inflation in this class of models.Physic

Flavoured large N gauge theory on a compact space with an external magnetic field

The phase structure of flavoured N = 2 SYM on a three sphere in an external magnetic field is studied. The pairing effect of the magnetic field competes with the dissociating effect of the Casimir energy, leading to an interesting phase structure of confined and deconfined phases separated by a critical curve of a first order quantum phase transition. At vanishing magnetic field the phase transition is of a third order. For sufficiently strong magnetic field, the only stable phase is the confined phase and magnetic catalysis of chiral symmetry breaking is realized. The meson spectra of the theory exhibit Zeeman splitting and level crossing and feature a finite jump at the phase transition between the confined and deconfined phases. At strong magnetic field the ground state has a massless mode corresponding to the Goldstone boson associated with the spontaneously broken U(1) R-symmetry analogous to the η' meson in QCD

Supermanifolds - Application to Supersymmetry

Parity is ubiquitous, but not always identified as a simplifying tool for computations. Using parity, having in mind the example of the bosonic/fermionic Fock space, and the framework of Z_2-graded (super) algebra, we clarify relationships between the different definitions of supermanifolds proposed by various people. In addition, we work with four complexes allowing an invariant definition of divergence: - an ascending complex of forms, and a descending complex of densities on real variables - an ascending complex of forms, and descending complex of densities on Grass mann variables. This study is a step towards an invariant definition of integrals of superfunctions defined on supermanifolds leading to an extension to infinite dimensions. An application is given to a construction of supersymmetric Fock spaces.Comment: to appear in the "Michael Marinov Memorial Volume

Towards Relativistic Skyrmions

We revisit baryons in the Skyrme model. Starting from static baryons in the helicity eigenstates, we generalize their wavefunctions to the non-static and relativistic regime. A new representation for gamma matrices in the soliton collective space is constructed and the corresponding Dirac equation is obtained. As an example, we draw consideration on how to apply this new representation on the calculus of vector current vacuum expectation values for baryon states of spin and isospin half and arbitrary momenta and we show how elastic form factors can be derived.Comment: 7 pages, 7th Conference Mathematical Methods in Physics - Londrina 2012, Rio de Janeiro, Brazi

Dynamics of Non-supersymmetric Flavours

We continue investigating the effect of the back-reaction by non-supersymmetric probes in the Kuperstein-Sonnenschein model. In the limit where the back-reaction is small, we discuss physical properties of the back-reacted geometry. We further introduce additional probe flavours in this back-reacted geometry and study in detail the phase structure of this sector when a constant electromagnetic field or a chemical potential are present. We find that the Landau pole, which serves as the UV cut-off of the background geometry, also serves as an important scale in the corresponding thermodynamics of the additional flavour sector. We note that since these additional probe flavours are indistinguishable from the back-reacting flavours, the results we obtain point to a much richer phase structure of the system

Production of negative parity baryons in the holographic Sakai-Sugimoto model

We extend our investigation of resonance production in the Sakai-Sugimoto model to the case of negative parity baryon resonances. Using holographic techniques we extract the generalized Dirac and Pauli baryon form factors as well as the helicity amplitudes for these baryonic states. Identifying the first negative parity resonance with the experimentally observed S_{11}(1535), we find reasonable agreement with experimental data from the JLab-CLAS collaboration. We also estimate the contribution of negative parity baryons to the proton structure functions.Comment: 1+30 pages, 8 figures. v3: typos corrected, references added, section 2 and 3 shortened, improved discussion of helicity amplitudes and comparison with experimental result

Generalized baryon form factors and proton structure functions in the Sakai-Sugimoto model

We investigate the production of positive parity baryon resonances in proton electromagnetic scattering within the Sakai-Sugimoto model. The latter is a string model for the non-perturbative regime of large $N_c$ QCD. Using holographic techniques we calculate the generalized Dirac and Pauli form factors that describe resonance production. We use these results to estimate the contribution of resonance production to the proton structure functions. Interestingly, we find an approximate Callan-Gross relation for the structure functions in a regime of intermediate values of the Bjorken variable.Comment: v3: 38 pages, 25 figures. We added a new subsection in order to compare our results with experimental data. To appear in Nucl. Phys.