480 research outputs found

### Zero Temperature Limit of Holographic Superconductors

We consider holographic superconductors whose bulk description consists of
gravity minimally coupled to a Maxwell field and charged scalar field with
general potential. We give an analytic argument that there is no "hard gap":
the real part of the conductivity at low frequency remains nonzero (although
typically exponentially small) even at zero temperature. We also numerically
construct the gravitational dual of the ground state of some holographic
superconductors. Depending on the charge and dimension of the condensate, the
infrared theory can have emergent conformal or just Poincare symmetry. In all
cases studied, the area of the horizon of the dual black hole goes to zero in
the extremal limit, consistent with a nondegenerate ground state.Comment: 27 pages, 8 figure

### The gauge-string duality and heavy ion collisions

I review at a non-technical level the use of the gauge-string duality to
study aspects of heavy ion collisions, with special emphasis on the trailing
string calculation of heavy quark energy loss. I include some brief
speculations on how variants of the trailing string construction could provide
a toy model of black hole formation and evaporation. This essay is an invited
contribution to "Forty Years of String Theory" and is aimed at philosophers and
historians of science as well as physicists.Comment: 21 page

### Non-supersymmetric deformations of the dual of a confining gauge theory

We introduce a computational technique for studying non-supersymmetric
deformations of domain wall solutions of interest in AdS/CFT. We focus on the
Klebanov-Strassler solution, which is dual to a confining gauge theory. From an
analysis of asymptotics we find that there are three deformations that leave
the ten-dimensional supergravity solution regular and preserve the global
bosonic symmetries of the supersymmetric solution. Also, we show that there are
no regular near-extremal deformations preserving the global symmetries, as one
might expect from the existence of a gap in the gauge theory.Comment: 18 pages, latex, published as JHEP 0305 (2003) 03

### The Gregory-Laflamme instability for the D2-D0 bound state

The D2-D0 bound state exhibits a Gregory-Laflamme instability when it is
sufficiently non-extremal. If there are no D0-branes, the requisite
non-extremality is finite. When most of the extremal mass comes from D0-branes,
the requisite non-extremality is very small. The location of the threshhold for
the instability is determined using a local thermodynamic analysis which is
then checked against a numerical analysis of the linearized equations of
motion. The thermodynamic analysis reveals an instability of non-commutative
field theory at finite temperature, which may occur only at very long
wavelengths as the decoupling limit is approached.Comment: 19 pages, Latex2e. v2: two refs added. v3: clearer exposition in
section

### Asymptotic Symmetry and the General Black Hole Solution in Ads_3 Gravity

We describe the Brown-Henneaux asymptotic symmetry of the general black holes
in the Chern-Simons gauge theory of the gauge group $SL(2;{\bf R})_L\times
SL(2;{\bf R})_R$. We make it clear that the vector-like subgroup $SL(2; {\bf
R})_{L+R}$ plays an essential role in describing the asymptotic symmetry
consistently. We find a quite general black hole solution in the $AdS_3$
gravity theory. The solution is specified by an infinite number of conserved
quantities which constitute a family of mapping from $S^1$ to the gauge group.
The BTZ black hole is one of the simplest case.Comment: 19 pages, no figur

### On (multi-)center branes and exact string vacua

Multicenter supergravity solutions corresponding to Higgs phases of
supersymmetric Yang-Mills theories are considered. For NS5 branes we identify
three cases where there is a description in terms of exact conformal field
theories. Other supergravity solutions, such as D3-branes with angular
momentum, are understood as special limits of multicenter ones. Within our
context we also consider 4-dim gravitational multi-instantons.Comment: 9 pages, latex; To be published in the proceedings of the Quantum
Aspects of Gauge Theories, Supersymmetry and Unification, Corfu, Greece,
20-26 September 199

### Introduction to Holographic Superconductors

These lectures give an introduction to the theory of holographic
superconductors. These are superconductors that have a dual gravitational
description using gauge/gravity duality. After introducing a suitable
gravitational theory, we discuss its properties in various regimes: the probe
limit, the effects of backreaction, the zero temperature limit, and the
addition of magnetic fields. Using the gauge/gravity dictionary, these
properties reproduce many of the standard features of superconductors. Some
familiarity with gauge/gravity duality is assumed. A list of open problems is
included at the end.Comment: 34 pages, 10 figures, to appear in the proceedings of the 5th Aegean
Summer School, "From Gravity to Thermal Gauge Theories: the AdS/CFT
Correspondence"; v2: references adde

### Heavy-quark energy loss in pQCD and SYM plasmas

We consider heavy-quark energy loss and pT-broadening in a strongly-coupled
N=4 Super Yang Mills (SYM) plasma, and the problem of finite-extend matter is
addressed. When expressed in terms of the appropriate saturation momentum, one
finds identical parametric forms for the energy loss in pQCD and SYM theory,
while pT-broadening is radiation dominated in SYM theory and multiple
scattering dominated in pQCD.Comment: 5 pages, 2 figures, Proceedings of the IIIrd Workshop for Young
Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions
(HotQuarks08), Estes Park, USA, August 18-23 200

### Robustness of Sound Speed and Jet Quenching for Gauge/Gravity Models of Hot QCD

We probe the effectiveness and robustness of a simple gauge/gravity dual
model of the QCD fireball that breaks conformal symmetry by constructing a
family of similar geometries that solve the scalar/gravity equations of motion.
This family has two parameters, one of which is associated to the temperature.
We calculate two quantities, the speed of sound and the jet-quenching
parameter. We find the speed of sound to be universal and robust over all the
geometries when appropriate units are used, while the jet-quenching parameter
varies significantly away from the conformal limit. We note that the overall
structure of the jet-quenching depends strongly on whether the running scalar
is the dilaton or not. We also discuss the variation of the scalar potential
over our family of solutions, and truncate our results to where the associated
error is small.Comment: 21 pages, 9 figures, LaTeX. v2:references added, minor correction to
speed of sound; conclusions unchange

### Cascading RG Flows from New Sasaki-Einstein Manifolds

In important recent developments, new Sasaki-Einstein spaces $Y^{p,q}$ and
conformal gauge theories dual to $AdS_5\times Y^{p,q}$ have been constructed.
We consider a stack of N D3-branes and M wrapped D5-branes at the apex of a
cone over $Y^{p,q}$. Replacing the D-branes by their fluxes, we construct
asymptotic solutions for all p and q in the form of warped products of the cone
and $R^{3,1}$. We show that they describe cascading RG flows where N decreases
logarithmically with the scale. The warp factor, which we determine explicitly,
is a function of the radius of the cone and one of the coordinates on
$Y^{p,q}$. We describe the RG cascades in the dual quiver gauge theories, and
find an exact agreement between the supergravity and the field theory beta
functions. We also discuss certain dibaryon operators and their dual wrapped
D3-branes in the conformal case M=0.Comment: 22 pages, 6 figures; v2 minor corrections; v3 refs, orbifold
discussion added; v4 more ref

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