872 research outputs found
The viscosity bound in string theory
The ratio of shear viscosity to entropy density of any material in
nature has been conjectured to have a lower bound of , the famous KSS
bound. We examine string theory models for evidence in favour of and against
this conjecture. We show that in a broad class of models quantum corrections
yield values of just above the KSS bound. However, incorporating
matter fields in the fundamental representation typically leads to violations
of this bound. We also outline a program to extend AdS/CFT methods to RHIC
phenomenology.Comment: 4 pages, To appear in the conference proceedings for Quark Matter
2009, March 30 - April 4, Knoxville, Tennesse
Holographic bulk viscosity: GPR vs EO
Recently Eling and Oz (EO) proposed a formula for the holographic bulk
viscosity, in arXiv:1103.1657, derived from the null horizon focusing equation.
This formula seems different from that obtained earlier by Gubser, Pufu and
Rocha (GPR) in arXiv:0806.0407 calculated from the IR limit of the two-point
function of the trace of the stress tensor. The two were shown to agree only
for some simple scaling cases. We point out that the two formulae agree in two
non-trivial holographic theories describing RG flows. The first is the strongly
coupled N=2* gauge theory plasma. The second is the semi-phenomenological model
of Improved Holographic QCD.Comment: 21 pages, 2 figure
Deriving N=2 S-dualities from Scaling for Product Gauge Groups
S-dualities in scale invariant N=2 supersymmetric field theories with product
gauge groups are derived by embedding those theories in asymptotically free
theories with higher rank gauge groups. S-duality transformations on the
couplings of the scale invariant theory follow from the geometry of the
embedding of the scale invariant theory in the Coulomb branch of the
asymptotically free theory.Comment: 19 pages latex and 1 figure using psfig.sty. References adde
sQGP as hCFT
We examine the proposal to make quantitative comparisons between the strongly
coupled quark-gluon plasma and holographic descriptions of conformal field
theory. In this note, we calculate corrections to certain transport
coefficients appearing in second-order hydrodynamics from higher curvature
terms to the dual gravity theory. We also clarify how these results might be
consistently applied in comparisons with the sQGP.Comment: 13 page
Nonlocal probes of thermalization in holographic quenches with spectral methods
We describe the application of pseudo-spectral methods to problems of
holographic thermal quenches of relevant couplings in strongly coupled gauge
theories. We focus on quenches of a fermionic mass term in a strongly coupled
N=4 supersymmetric Yang-Mills plasma, and the subsequent equilibration of the
system. From the dual gravitational perspective, we study the gravitational
collapse of a massive scalar field in asymptotically anti-de Sitter geometry
with a prescribed boundary condition for its non-normalizable mode. Access to
the full background geometry of the gravitational collapse allows for the study
of nonlocal probes of the thermalization process. We discuss the evolution of
the apparent and the event horizons, the two-point correlation functions of
operators of large conformal dimensions, and the evolution of the entanglement
entropy of the system. We compare the thermalization process from the viewpoint
of local (the one-point) correlation functions and these nonlocal probes,
finding that the thermalization time as measured by the probes is length
dependent, and approaches the thermalization time of the one-point function for
longer probes. We further discuss how the different energy scales of the
problem contribute to its thermalization.Comment: 83 pages, 25 figures. v2: Corrected constraint in equation (A.26),
which led to non-monotonic apparent horizons in our simulations. Replaced
most figures. Added equation (4.11). Added references [37], [38]. Added
acknowledgement. Corrected some typos. Most conclusions remain unchange
Equilibration rates in a strongly coupled nonconformal quark-gluon plasma
We initiate the study of equilibration rates of strongly coupled quark-gluon
plasmas in the absence of conformal symmetry. We primarily consider a
supersymmetric mass deformation within gauge theory and use
holography to compute quasinormal modes of a variety of scalar operators, as
well as the energy-momentum tensor. In each case, the lowest quasinormal
frequency, which provides an approximate upper bound on the thermalization
time, is proportional to temperature, up to a pre-factor with only a mild
temperature dependence. We find similar behaviour in other holographic plasmas,
where the model contains an additional scale beyond the temperature. Hence, our
study suggests that the thermalization time is generically set by the
temperature, irrespective of any other scales, in strongly coupled gauge
theories.Comment: 6 pages, 7 figure
Holographic dual of the Eguchi-Kawai mechanism
archiveprefix: arXiv primaryclass: hep-th reportnumber: NORDITA-2014-40, UUITP-03-14, QMUL-PH-14-08 slaccitation: %%CITATION = ARXIV:1404.0225;%%archiveprefix: arXiv primaryclass: hep-th reportnumber: NORDITA-2014-40, UUITP-03-14, QMUL-PH-14-08 slaccitation: %%CITATION = ARXIV:1404.0225;%%archiveprefix: arXiv primaryclass: hep-th reportnumber: NORDITA-2014-40, UUITP-03-14, QMUL-PH-14-08 slaccitation: %%CITATION = ARXIV:1404.0225;%%The work of K.Z. was supported by
the ERC advanced grant No 341222, by the Marie Curie network GATIS of the European
Unionâs FP7 Programme under REA Grant Agreement No 317089, and by the Swedish
Research Council (VR) grant 2013-4329. DY acknowledges NORDITA where this work
was begun, during his time as a NORDITA fellow
Quantum quenches of holographic plasmas
We employ holographic techniques to study quantum quenches at finite
temperature, where the quenches involve varying the coupling of the boundary
theory to a relevant operator with an arbitrary conformal dimension
2\leq\D\leq4. The evolution of the system is studied by evaluating the
expectation value of the quenched operator and the stress tensor throughout the
process. The time dependence of the new coupling is characterized by a fixed
timescale and the response of the observables depends on the ratio of the this
timescale to the initial temperature. The observables exhibit universal scaling
behaviours when the transitions are either fast or slow, i.e. when this ratio
is very small or very large. The scaling exponents are smooth functions of the
operator dimension. We find that in fast quenches, the relaxation time is set
by the thermal timescale regardless of the operator dimension or the precise
quenching rate.Comment: 60 pages, 10 figures, 3 appendice
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