122 research outputs found
Colliding waves on a string in AdS
This paper is concerned with the classical motion of a string in global
AdS. The initially static string stretches between two antipodal points on
the boundary circle. Both endpoints are perturbed which creates cusps at a
steady rate. The cusps propagate towards the interior where they collide. The
behavior of the string depends on the strength of forcing. Three qualitatively
different phases can be distinguished: transparent, gray, and black. The
transparent region is analogous to a standing wave. In the black phase, there
is a horizon on the worldsheet and cusps never reach the other endpoint. The
string keeps folding and its length grows linearly over time. In the gray
phase, the string still grows linearly. However, cusps do cross to the other
side. The transparent and gray regions are separated by a transition point
where a logarithmic accumulation of cusps is numerically observed. A simple
model reproduces the qualitative behavior of the string in the three phases.Comment: 9 pages, 16 figure
Holography without translational symmetry
We propose massive gravity as a holographic framework for describing a class
of strongly interacting quantum field theories with broken translational
symmetry. Bulk gravitons are assumed to have a Lorentz-breaking mass term as a
substitute for spatial inhomogeneities. This breaks momentum-conservation in
the boundary field theory. At finite chemical potential, the gravity duals are
charged black holes in asymptotically anti-de Sitter spacetime. The
conductivity in these systems generally exhibits a Drude peak that approaches a
delta function in the massless gravity limit. Furthermore, the optical
conductivity shows an emergent scaling law: . This result is consistent with that found earlier by
Horowitz, Santos, and Tong who introduced an explicit inhomogeneous lattice
into the system.Comment: 8 pages, 3 figures; v2: minor correction
Moduli spaces of gauge theories from dimer models: Proof of the correspondence
Recently, a new way of deriving the moduli space of quiver gauge theories that arise on the world–volume of D3–branes probing singular toric Calabi–Yau cones was conjectured. According to the proposal, the gauge group, matter content and tree–level superpotential of the gauge theory is encoded in a periodic tiling, the dimer graph. The conjecture provides a simple procedure for determining the moduli space of the gauge theory in terms of perfect matchings. For gauge theories described by periodic quivers that can be embedded on a two–dimensional torus, we prove the equivalence between the determination of the toric moduli space with a gauged linear sigma model and the computation of the Newton polygon of the characteristic polynomial of the dimer model. We show that perfect matchings are in one–to–one correspondence with fields in the linear sigma model. Furthermore, we prove that the position in the toric diagram of every sigma model field is given by the slope of the height function of the corresponding perfect matching
Non-Fermi liquids from holography
We report on a potentially new class of non-Fermi liquids in
(2+1)-dimensions. They are identified via the response functions of composite
fermionic operators in a class of strongly interacting quantum field theories
at finite density, computed using the AdS/CFT correspondence. We find strong
evidence of Fermi surfaces: gapless fermionic excitations at discrete shells in
momentum space. The spectral weight exhibits novel phenomena, including
particle-hole asymmetry, discrete scale invariance, and scaling behavior
consistent with that of a critical Fermi surface postulated by Senthil.Comment: 10 pages, 16 figures. v2: added references, corrected figures, some
minor changes. v3: figure 5 replace
Branes, graphs and singularities
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 341-354).In this thesis, we study various aspects of string theory on geometric and nongeometric backgrounds in the presence of branes. In the first part of the thesis, we study non-compact geometries. We introduce "brane tilings" which efficiently encode the gauge group, matter content and superpotential of various quiver gauge theories that arise as low-energy effective theories for D-branes probing singular non-compact Calabi-Yau spaces with toric symmetries. Brane tilings also offer a generalization of the AdS/CFT correspondence. A technique is developed which enables one to quickly compute the toric vacuum moduli space of the quiver gauge theory. The equivalence of this procedure and the earlier approach that used gauged linear sigma models is explicitly shown. As an application of brane tilings, four dimensional quiver gauge theories are constructed that are AdS/CFT dual to infinite families of Sasaki-Einstein spaces. Various checks of the correspondence are performed. We then develop a procedure that constructs the brane tiling for an arbitrary toric Calabi-Yau threefold. This solves a longstanding problem by computing superpotentials for these theories directly from the toric diagram of the singularity. A different approach to the low-energy theory of D-branes uses exceptional collections of sheaves associated to the base of the threefold. We provide a dictionary that translates between the language of brane tilings and that of exceptional collections. Geometric compactifications represent only a very small subclass of the landscape: the generic vacua are non-geometric. In the second part of the thesis, we study perturbative compactifications of string theory that rely on a fibration structure of the extra dimensions. Non-geometric spaces preserving .A = 1 supersymmetry in four dimensions are obtained by using T-dualities as monodromies. Several examples are discussed, some of which admit an asymmetric orbifold description. We explore the possibility of twisted reductions where left-moving spacetime fermion number Wilson lines are turned on in the fiber.by David Vegh.Ph.D
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