315 research outputs found
Shear viscosity from R-charged AdS black holes
We compute the shear viscosity in the supersymmetric Yang-Mills theory dual
to the STU background. This is a thermal gauge theory with a chemical
potential. The quotient of the shear viscosity over the entropy density
exhibits no deviation from the well known result 1/4\pi.Comment: 9 pages, some references updated, abstract and some typos correcte
Low frequency quasi-normal modes of AdS black holes
We calculate analytically low frequency quasi-normal modes of gravitational
perturbations of AdS Schwarzschild black holes in dimensions. We arrive at
analytic expressions which are in agreement with their counterparts from
linearized hydrodynamics in , in accordance with the
AdS/CFT correspondence. Our results are also in good agreement with results of
numerical calculations.Comment: 14 page
Operator mixing and three-point functions in N=4 SYM
We study the three-point functions between two BPS and one non-BPS local
gauge invariant operators in N=4 Super Yang-Mills theory. In particular we
show, in explicit 1-loop examples, that the operator mixing discussed in
arXiv:0810.0499 plays an important role in the computations of the correlators
and is necessary to cancel contributions that would violate the constraints
following from the superconformal and the bonus U(1)_Y symmetries. We analyse
the same type of correlators also at strong coupling by using the BMN limit of
the AdS_5xS^5 string theory. Again the mixing between states with different
types of impurities is crucial to ensure the cancellation of various amplitudes
that would violate the constraints mentioned above. However, on the string
side, we also find some examples of interactions between one non-BPS and two
BPS states that do not satisfy expectations based on the superconformal and the
bonus U(1)_Y symmetries.Comment: 28 pages, 5 figure
Ehlers symmetry at the next derivative order
We analyse four-dimensional gravity in the presence of general curvature
squared corrections and show that Ehlers' SL(2,R) symmetry, which appears in
the reduction of standard gravity to three dimensions, is preserved by the
correction terms. The mechanism allowing this is a correction of the SL(2,R)
transformation laws which resolves problems with the different scaling
behaviour of various terms occurring in the reduction.Comment: 13 pages. v2: updated referenc
Non-Invasive Estimation of Right Atrial Pressure Using a Semi-Automated Echocardiographic Tool for Inferior Vena Cava Edge-Tracking
The non-invasive estimation of right atrial pressure (RAP) would be a key advancement in several clinical scenarios, in which the knowledge of central venous filling pressure is vital for patientsâ management. The echocardiographic estimation of RAP proposed by Guidelines, based on inferior vena cava (IVC) size and respirophasic collapsibility, is exposed to operator and patient dependent variability. We propose novel methods, based on semi-automated edge-tracking of IVC size and cardiac collapsibility (cardiac caval indexâCCI), tested in a monocentric retrospective cohort of patients undergoing echocardiography and right heart catheterization (RHC) within 24 h in condition of clinical and therapeutic stability (170 patients, age 64 ± 14, male 45%, with pulmonary arterial hypertension, heart failure, valvular heart disease, dyspnea, or other pathologies). IVC size and CCI were integrated with other standard echocardiographic features, selected by backward feature selection and included in a linear model (LM) and a support vector machine (SVM), which were cross-validated. Three RAP classes (low 10 mmHg) were generated and RHC values used as comparator. LM and SVM showed a higher accuracy than Guidelines (63%, 71%, and 61% for LM, SVM, and Guidelines, respectively), promoting the integration of IVC and echocardiographic features for an improved non-invasive estimation of RAP
A note on the quartic effective action of type IIB superstring
We recast the result of our recent computation of the quartic action of
ten-dimensional IIB supergravity in a manifestly SL(2,R)-covariant form. This
affords us a critical assessment of an earlier conjecture in the literature.Comment: 15 pages; v2: some clarifications and a rerence adde
String Theory and Quantum Chromodynamics
I review recent progress on the connection between string theory and quantum
chromodynamics in the context of the gauge/gravity duality. Emphasis is placed
on conciseness and conceptual aspects rather than on technical details. Topics
covered include the large-Nc limit of gauge theories, the gravitational
description of gauge theory thermodynamics and hydrodynamics, and
confinement/deconfinement thermal phase transitions.Comment: 38 pages, 24 figures. Lectures given at the RTN Winter School on
"Strings, Supergravity and Gauge Theories" at CERN on January 15-19, 200
Lectures on hydrodynamic fluctuations in relativistic theories
These are pedagogical lecture notes on hydrodynamic fluctuations in normal
relativistic fluids. The lectures discuss correlation functions of conserved
densities in thermal equilibrium, interactions of the hydrodynamic modes, an
effective action for viscous fluids, and the breakdown of the derivative
expansion in hydrodynamics.Comment: 55 pages. Based on lectures given at the Seattle INT Summer School on
Applications of String Theory in July 201
Einstein-Maxwell gravitational instantons and five dimensional solitonic strings
We study various aspects of four dimensional Einstein-Maxwell multicentred
gravitational instantons. These are half-BPS Riemannian backgrounds of minimal
N=2 supergravity, asymptotic to R^4, R^3 x S^1 or AdS_2 x S^2. Unlike for the
Gibbons-Hawking solutions, the topology is not restricted by boundary
conditions. We discuss the classical metric on the instanton moduli space. One
class of these solutions may be lifted to causal and regular multi `solitonic
strings', without horizons, of 4+1 dimensional N=2 supergravity, carrying null
momentum.Comment: 1+30 page
- âŠ