899 research outputs found
Thermodynamics of AdS/QCD
We study finite temperature properties of four dimensional QCD-like gauge
theories in the gauge theory/gravity duality picture. The gravity dual contains
two deformed 5d AdS metrics, with and without a black hole, and a dilaton. We
study the thermodynamics of the 4d boundary theory and constrain the two
metrics so that they correspond to a high and a low temperature phase separated
by a first order phase transition. The equation of state has the standard form
for the pressure of a strongly coupled fluid modified by a vacuum energy, a bag
constant. We determine the parameters of the deformation by using QCD results
for and the hadron spectrum. With these parameters, we show that the
phase transition in the 4d boundary theory and the 5d bulk Hawking-Page
transition agree. We probe the dynamics of the two phases by computing the
quark-antiquark free energy in them and confirm that the transition corresponds
to confinement-deconfinement transition.Comment: 1+19 pages, 6 figures, references added, section 3 improve
Short distance properties of cascading gauge theories
We study the short distance (large momentum) properties of correlation
functions of cascading gauge theories by performing a tree-level computation in
their dual gravitational background. We prove that these theories are
holographically renormalizable; the correlators have only analytic ultraviolet
divergences, which may be removed by appropriate local counterterms. We find
that n-point correlation functions of properly normalized operators have the
expected scaling in the semi-classical gravity (large N) limit: they scale as
N_{eff}^{2-n} with N_{eff} proportional to ln(k/Lambda) where k is a typical
momentum. Our analysis thus confirms the interpretation of the cascading gauge
theories as renormalizable four-dimensional quantum field theories with an
effective number of degrees of freedom which logarithmically increases with the
energy.Comment: 47 pages, no figure
Sound Waves in (2+1) Dimensional Holographic Magnetic Fluids
We use the AdS/CFT correspondence to study propagation of sound waves in
strongly coupled (2+1) dimensional conformal magnetic fluids. Our computation
provides a nontrivial consistency check of the viscous magneto-hydrodynamics of
Hartnoll-Kovtun-Muller-Sachdev to leading order in the external field.
Depending on the behavior of the magnetic field in the hydrodynamic limit, we
show that it can lead to further attenuation of sound waves in the (2+1)
dimensional conformal plasma, or reduce the speed of sound. We present both
field theory and dual supergravity descriptions of these phenomena. While to
the leading order in momenta the dispersion of the sound waves obtained from
the dual supergravity description agrees with the one predicted from field
theory, we find a discrepancy at higher order. This suggests that further
corrections to HKMS magneto-hydrodynamics are necessary.Comment: 32 pages, LaTeX; minor corrections, references added; an error in the
boundary conditions fixed, a slight change in the result
Fourth sound of holographic superfluids
We compute fourth sound for superfluids dual to a charged scalar and a gauge
field in an AdS_4 background. For holographic superfluids with condensates that
have a large scaling dimension (greater than approximately two), we find that
fourth sound approaches first sound at low temperatures. For condensates that a
have a small scaling dimension it exhibits non-conformal behavior at low
temperatures which may be tied to the non-conformal behavior of the order
parameter of the superfluid. We show that by introducing an appropriate scalar
potential, conformal invariance can be enforced at low temperatures.Comment: 20 pages, 6 figures; removed a discussion on a variant of fourth
soun
Leading strategies in competitive on-line prediction
We start from a simple asymptotic result for the problem of on-line
regression with the quadratic loss function: the class of continuous
limited-memory prediction strategies admits a "leading prediction strategy",
which not only asymptotically performs at least as well as any continuous
limited-memory strategy but also satisfies the property that the excess loss of
any continuous limited-memory strategy is determined by how closely it imitates
the leading strategy. More specifically, for any class of prediction strategies
constituting a reproducing kernel Hilbert space we construct a leading
strategy, in the sense that the loss of any prediction strategy whose norm is
not too large is determined by how closely it imitates the leading strategy.
This result is extended to the loss functions given by Bregman divergences and
by strictly proper scoring rules.Comment: 20 pages; a conference version is to appear in the ALT'2006
proceeding
Brane-Inspired Orientifold Field Theories
In this paper we consider the gauge theory living on the world-volume of a
stack of N D3-branes of Type 0B/\Omega' I_6(-1)^{F_{L}} and of its orbifolds
C^2/Z_2 and C^3/(Z_2 x Z_2). The gauge theories obtained in the three cases are
a brane realization of ``orientifold field theories'' having the bosonic sector
common with N=4,2,1 super Yang-Mills respectively. In these non-supersymmetric
theories, we investigate the possibility of keeping the gauge/gravity
correspondence that has revealed itself so successful in the case of
supersymmetric theories. In the open string framework we compute the
coefficient of the gauge kinetic term showing that the perturbative behaviour
of the orientifold field theory can be obtained from the closed string channel
in the large N limit, where the theory exhibits Bose-Fermi degeneracy.Comment: 27 pages, LaTe
Gauge/Gravity Correspondence from Open/Closed String Duality
We compute the annulus diagram corresponding to the interaction of a
fractional D3 brane with a gauge field on its world-volume and a stack of N
fractional D3 branes on the orbifolds C^2 /Z_2 and C^3/Z_2 x Z_2. We show that
its logarithmic divergence can be equivalently understood as due either to
massless open string states circulating in the loop or to massless closed
string states exchanged between two boundary states. This follows from the fact
that, under open/closed string duality, massless states in the open and closed
string channels are matched into each other without mixing with massive states.
This explains why the perturbative properties of many gauge theories living on
the worldvolume of less supersymmetric and nonconformal branes have been
recently obtained from their corresponding supergravity solution.Comment: LaTeX, 28 page
Unoriented D-brane Instantons vs Heterotic worldsheet Instantons
We discuss Fermi interactions of four hyperini generated by ``stringy''
instantons in a Type I / Heterotic dual pair on T^4/Z_2.Comment: Minor corrections and clarifications. Added reference
Real-time gauge/gravity duality: Prescription, Renormalization and Examples
We present a comprehensive analysis of the prescription we recently put
forward for the computation of real-time correlation functions using
gauge/gravity duality. The prescription is valid for any holographic
supergravity background and it naturally maps initial and final data in the
bulk to initial and final states or density matrices in the field theory. We
show in detail how the technique of holographic renormalization can be applied
in this setting and we provide numerous illustrative examples, including the
computation of time-ordered, Wightman and retarded 2-point functions in
Poincare and global coordinates, thermal correlators and higher-point
functions.Comment: 85 pages, 13 figures; v2: added comments and reference
Holographic dual of the Standard Model on the throat
We apply recent techniques to construct geometries, based on local Calabi-Yau
manifolds, leading to warped throats with 3-form fluxes in string theory, with
interesting structure at their bottom. We provide their holographic dual
description in terms of RG flows for gauge theories with almost conformal
duality cascades and infrared confinement. We describe a model of a throat with
D-branes at its bottom, realizing a 3-family Standard Model like chiral sector.
We provide the explicit holographic dual gauge theory RG flow, and describe the
appearance of the SM degrees of freedom after confinement. As a second
application, we describe throats within throats, namely warped throats with
discontinuous warp factor in different regions of the radial coordinate, and
discuss possible model building applications.Comment: 46 pages, 21 figures, reference adde
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