1,474 research outputs found
Thermodynamics of black branes in asymptotically Lifshitz spacetimes
Recently, a class of gravitational backgrounds in 3+1 dimensions have been
proposed as holographic duals to a Lifshitz theory describing critical
phenomena in 2+1 dimensions with critical exponent . We continue our
earlier work \cite{Bertoldi:2009vn}, exploring the thermodynamic properties of
the "black brane" solutions with horizon topology . We find that
the black branes satisfy the relation where
is the energy density, is the temperature, and is the
entropy density. This matches the expected behavior for a 2+1 dimensional
theory with a scaling symmetry , .Comment: 8 pages, references added and regroupe
Evaluation of intravoxel incoherent motion fitting methods in low-perfused tissue
Purpose To investigate the robustness of constrained and simultaneous intravoxel incoherent motion (IVIM) fitting methods and the estimated IVIM parameters (D, D* and f) for applications in brain and lowâperfused tissues. Materials and Methods Model data simulations relevant to brain and lowâperfused tumor tissues were computed to assess the accuracy, relative bias, and reproducibility (CV%) of the fitting methods in estimating the IVIM parameters. The simulations were performed at a series of signalâtoânoise ratio (SNR) levels to assess the influence of noise on the fitting. Results The estimated IVIM parameters from model simulations were found significantly different (Pâ<â0.05) using simultaneous and constrained fitting methods at low SNR. Higher accuracy and reproducibility were achieved with the constrained fitting method. Using this method, the mean error (%) for the estimated IVIM parameters at a clinically relevant SNRâ=â40 were D 0.35, D* 41.0 and f 4.55 for the tumor model and D 1.87, D* 2.48, and f 7.49 for the gray matter model. The most robust parameters were the IVIMâD and IVIMâf. The IVIMâD* was increasingly overestimated at low perfusion. Conclusion A constrained IVIM fitting method provides more accurate and reproducible IVIM parameters in lowâperfused tissue compared with simultaneous fitting. Level of Evidence:
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Fighting HIV/AIDS: Reconfiguring the State?
The author wishes to thank the anonymous reviewers of the article and the ESRC for funding part of this research
Brane Baldness vs. Superselection Sectors
The search for intersecting brane solutions in supergravity is a large and
profitable industry. Recently, attention has focused on finding localized forms
of known `delocalized' solutions. However, in some cases, a localized version
of the delocalized solution simply does not exist. Instead, localized separated
branes necessarily delocalize as the separation is removed. This phenomenon is
related to black hole no-hair theorems, i.e. `baldness.' We continue the
discussion of this effect and describe how it can be understood, in the case of
Dirichlet branes, in terms of the corresponding intersection field theory. When
it occurs, it is associated with the quantum mixing of phases and lack of
superselection sectors in low dimensional field theories. We find surprisingly
wide agreement between the field theory and supergravity both with respect to
which examples delocalize and with respect to the rate at which this occurs.Comment: 26 pages, ReVTeX, 2 figures, reference added, version to appear in
PR
Effect of Scalar Mass in the Absorption and Emission Spectra of Schwarzschild Black Hole
Following Sanchez's approach we investigate the effect of scalar mass in the
absorption and emission problems of 4d Schwarzschild black hole. The absorption
cross sections for arbitrary angular momentum of the scalar field are computed
numerically in the full range of energy by making use of the analytic
near-horizon and asymptotic solutions and their analytic continuations. The
scalar mass makes an interesting effect in the low-energy absorption cross
section for S-wave. Unlike the massless case, the cross section decreases with
increasing energy in the extremely low-energy regime. As a result the
universality, {\it i.e.} low-energy cross section for S-wave is equal to the
horizon area, is broken in the presence of mass. If the scalar mass is larger
than a critical mass, the absorption cross section becomes monotonically
decreasing function in the entire range of energy. The Hawking emission is also
calculated numerically. It turns out that the Planck factor generally
suppresses the contribution of higher partial waves except S-wave. The scalar
mass in general tends to reduce the emission rate.Comment: 20 pages, 8 figures, V2: few comments are added. version to appear in
Classical and Quantum Gravit
Exact Renormalization Group and Loop Equation
We propose a gauge invariant formulation of the exact renormalization group
equation for nonsupersymmetric pure U(N) Yang-Mills theory, based on the
construction by Tim Morris. In fact we show that our renormalization group
equation amounts to a regularized version of the loop equation, thereby
providing a direct relation between the exact renormalization group and the
Schwinger-Dyson equations. We also discuss a possible implication of our
formulation to the holographic correspondence of the bulk gravity and the
boundary gauge theory.Comment: 13 pages, Latex, References added. An error in eq. (6) fixed and a
few corrrections accordingly. Results unchange
Holographic description of D3-branes in flat space
We describe a scheme for constructing the holographic dual of the full
D3-brane geometry with charge by embedding it into a large anti-de Sitter
space of size . Such a geometry is realized in a multi-center anti-de Sitter
geometry which admits a simple field theory interpretation as gauge
theory broken to . We find that the characteristic size of
the D3-brane geometry is of order where is the scale of
the Higgs. By choosing to be much larger than , the scale of the
D3-brane metric can be well separated from the Higgs scale in the radial
coordinate. We generalize the holographic energy-distance relation and estimate
the characteristic energy scale associated with these radial scales, and find
that the relation becomes effectively independent in the range
. This implies that all detailed structure of the
D3-brane geometry is encoded in the fine structure of the boundary gauge theory
at around the Higgs scale.Comment: 8 pages, 1 eps figur
Black Holes and Five-brane Thermodynamics
The phase diagram for Dp-branes in M-theory compactified on , ,
, and is constructed. As for the lower-dimensional tori considered
in our previous work (hep-th/9810224), the black brane phase at high entropy
connects onto matrix theory at low entropy; we thus recover all known instances
of matrix theory as consequences of the Maldacena conjecture. The difficulties
that arise for are reviewed. We also analyze the D1-D5 system on ;
we exhibit its relation to matrix models of M5-branes, and use spectral flow as
a tool to investigate the dependence of the phase structure on angular
momentum.Comment: 57 pages, 6 eps figures, latex. v2: DLCQ limit of 5-brane corrected;
typos corrected, references added. v3: reference added, typos corrected v4:
comments on DLCQ limit of 5-brane corrected one last time. Final version, to
appear in Phys. Rev.
On solvable models of type IIB superstring in NS-NS and R-R plane wave backgrounds
We consider type IIB string in the two plane-wave backgrounds which may be
interpreted as special limits of the AdS_3 x S^3 metric supported by either the
NS-NS or R-R 3-form field. The NS-NS plane-wave string model is equivalent to a
direct generalization of the Nappi-Witten model, with its spectrum being
similar to that of strings in constant magnetic field. The R-R model can be
solved in the light-cone gauge, where the Green-Schwarz action describes 4
massive and 4 massless copies of free bosons and fermions. We find the spectra
of the two string models and study the asymptotic density of states. We also
discuss a more general class of exactly solvable plane-wave models with reduced
supersymmetry which is obtained by adding twists in two spatial 2-planes.Comment: 36 pages, harvmac. v2: discussion of equivalence of the supergravity
parts of the spectra of the NS-NS and R-R models added in sect.5.3; v3: added
remark on periodicity of the NS-NS spectrum; v4: minor correction in sect.6.
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