5,012 research outputs found

### Using string theory to study the quark-gluon plasma: progress and perils

I outline some of the progress over the past few years in applying ideas from
string theory to study the quark-gluon plasma, including the computation of the
drag force on heavy quarks and estimates of total multiplicity from black hole
formation. I also indicate some of the main perils of the string theory
approach.Comment: 8 pages, 5 figures, contribution to the proceedings of Quark Matter
2009. v2: Slightly shortene

### Thermodynamics of spinning D3-branes

Spinning black three-branes in type IIB supergravity are thermodynamically
stable up to a critical value of the angular momentum density. Inside the
region of thermodynamic stability, the free energy from supergravity is roughly
reproduced by a naive model based on free N=4 super-Yang-Mills theory on the
world-volume. The field theory model correctly predicts a limit on angular
momentum density, but near this limit it does not reproduce the critical
exponents one can compute from supergravity. Analogies with Bose condensation
and modified matrix models are discussed, and a mean field theory improvement
of the naive model is suggested which corrects the critical exponents.Comment: 20 pages, 1 figure, small improvement

### Black hole absorption cross-sections and the anti-de Sitter -- conformal field theory correspondence

Recent work has uncovered a correspondence between theories in anti-de Sitter
space, and those on its boundary. This has important implications for black
holes in string theory which have near-horizon AdS geometries. Using the
effective coupling to the boundary conformal field theory, I compute the
low-energy, s-wave absorption cross-sections for a minimally coupled scalar in
the near-extremal four- and five-dimensional black holes. The results agree
precisely with semi-classical gravity calculations. Agreement for fixed
scalars, and for the BTZ black hole, is also found.Comment: 10 pages, harvma

### Low temperature properties of holographic condensates

In the current work we study various models of holographic superconductors at
low temperature. Generically the zero temperature limit of those models are
solitonic solution with a zero sized horizon. Here we generalized simple
version of those zero temperature solutions to small but non-zero temperature
T. We confine ourselves to cases where near horizon geometry is AdS^4. At a
non-zero temperature a small horizon would form deep inside this AdS^4 which
does not disturb the UV physics. The resulting geometry may be matched with the
zero temperature solution at an intermediate length scale. We understand this
matching from separation of scales by setting up a perturbative expansion in
gauge potential. We have a better analytic control in abelian case and
quantities may be expressed in terms of hypergeometric function. From this we
calculate low temperature behavior of various quatities like entropy, charge
density and specific heat etc. We also calculate various energy gaps associated
with p-wave holographic superconductor to understand the underlying pairing
mechanism. The result deviates significantly from the corresponding weak
coupling BCS counterpart.Comment: 17 Page

### Temperature of D3-branes off extremality

We discuss non-extremal rotating D3-branes. We solve the wave equation for
scalars in the supergravity background of certain distributions of branes and
compute the absorption coefficients. The form of these coefficients is similar
to the gray-body factors associated with black-hole scattering. They are given
in terms of two different temperature parameters, indicating that fields (open
string modes) do not remain in thermal equilibrium as we move off extremality.
This should shed some light on the origin of the disagreement between the
supergravity and conformal field theory results on the free energy of a system
of non-coincident D-branes.Comment: 15 pages in ReVTeX, incl. 1 figure (eepic

### Analytic evidence for the Gubser-Mitra conjecture

A simple master equation for the static perturbation of charged black strings
is derived while employing the gauge proposed by Kol. As the charge is varied
it is found that the potential in the master equation for the perturbations
becomes positive exactly when the specific heat turns positive thus forbidding
a bound state and the onset of the Gregory-Laflamme instability. It can safely
be said that this is the first analytic and explicit evidence for the
Gubser-Mitra conjecture, correlating the classical and thermodynamic
instabilities of black branes. Possible generalizations of the analysis are
also discussed.Comment: 9 pages, 2 figures, typos fixed, to appear in Phys. Lett.

### String Expansion as Large N Expansion of Gauge Theories

We consider string perturbative expansion in the presence of D-branes
imbedded in orbifolded space-time. In the regime where the string coupling is
weak and $\alpha'\to 0$, the string perturbative expansion coincides with `t
Hooft's large N expansion. We specifically concentrate on theories with d=4 and
${\cal N}=0,1,2,4$, and use world-sheet orbifold techniques to prove vanishing
theorems for the field theory beta functions to all orders in perturbation
theory in the large N limit. This is in accord with recent predictions.Comment: 18 pages, harvmac, minor corrections and additional reference

### A universal result on central charges in the presence of double-trace deformations

We study large N conformal field theories perturbed by relevant double-trace
deformations. Using the auxiliary field trick, or Hubbard-Stratonovich
transformation, we show that in the infrared the theory flows to another CFT.
The generating functionals of planar correlators in the ultraviolet and
infrared CFT's are shown to be related by a Legendre transform. Our main result
is a universal expression for the difference of the scale anomalies between the
ultraviolet and infrared fixed points, which is of order 1 in the large N
expansion. Our computations are entirely field theoretic, and the results are
shown to agree with predictions from AdS/CFT. We also remark that a certain
two-point function can be computed for all energy scales on both sides of the
duality, with full agreement between the two and no scheme dependence.Comment: 15 pages, latex2e, no figures. v2: references adde

### Strong Coupling Limit of ${\cal N}=2$ SCFT Free Energy and Higher Derivative AdS/CFT Correspondence

We study the role of higher derivative terms (Riemann curvature squared ones)
in thermodynamics of SCFTs via AdS/CFT correspondence. Using IIB string
effective action (d5 AdS gravity) with such HD terms deduced from heterotic
string via duality we calculate strong coupling limit of ${\cal N}=2$ SCFT free
energy with the account of next to leading term in large $N$ expansion. It is
compared with perturbative result following from boundary QFT. Considering
modification of such action where HD terms form Weyl squared tensor we found
(strong coupling limit) free energy in such theory. It is interesting that
leading and next to leading term of large $N$ expanded free energy may differ
only by factor 3/4 if compare with perturbative result. Considering HD gravity
as bosonic sector of some (compactified) HD supergravity we suggest new version
of AdS/CFT conjecture and successfully test it on the level of free energies
for ${\cal N}=2,4$ SCFTs.Comment: LaTeX file, 13 pages, misprints are correcte

### Holographic superconductors in the Born-Infeld electrodynamics

We study the effects of the Born-Infeld electrodynamics on the holographic
superconductors in the background of a Schwarzschild AdS black hole spacetime.
We find that the presence of Born-Infeld scale parameter decreases the critical
temperature and the ratio of the gap frequency in conductivity to the critical
temperature for the condensates. Our results means that it is harder for the
scalar condensation to form in the Born-Infeld electrodynamics.Comment: 9 pages, 2 figures, references added and corrected, the version
appears in PL

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