366 research outputs found
On the Temperature Dependence of the Shear Viscosity and Holography
We examine the structure of the shear viscosity to entropy density ratio
eta/s in holographic theories of gravity coupled to a scalar field, in the
presence of higher derivative corrections. Thanks to a non-trivial scalar field
profile, eta/s in this setup generically runs as a function of temperature. In
particular, its temperature behavior is dictated by the shape of the scalar
potential and of the scalar couplings to the higher derivative terms. We
consider a number of dilatonic setups, but focus mostly on phenomenological
models that are QCD-like. We determine the geometric conditions needed to
identify local and global minima for eta/s as a function of temperature, which
translate to restrictions on the signs and ranges of the higher derivative
couplings. Finally, such restrictions lead to an holographic argument for the
existence of a global minimum for eta/s in these models, at or above the
deconfinement transition.Comment: references adde
Scalar Spectrum from a Dynamical Gravity/Gauge model
We show that a Dynamical AdS/QCD model is able to reproduce the linear Regge
trajectories for the light-flavor sector of mesons with high spin and also for
the scalar and pseudoscalar ones. In addition the model has confinement by the
Wilson loop criteria and a mass gap. We also calculate the decay amplitude of
scalars into two pion in good agreement to the available experimental data.Comment: Presented in the 4th International Workshop on Astronomy and
Relativistic Astrophysic
Synthesis, in vitro antioxidant and antimicrobial activities of some novel 3-substitued-4-(3-methoxy-4-isobutyryloxybenzylideneamino)-4,5-dihydro- 1H-1,2,4-triazol-5-one derivatives
271-282In this study, 3-alkyl(aryl)-4-amino-4,5-dihydro-1H-1,2,4-triazol-5-ones have been reacted with 3-methoxy-4-isobutyryloxybenzaldehyde 2 to afford the corresponding nine new 3-alkyl(aryl)-4-(3-methoxy-4-isobutyryloxybenzylideneamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones 3. Then, the acetylation reactions of compounds 3 have been investigated and 4 type compounds have been obtained. The compounds 3 have also been treated with morpholine/1-methylpiperazine in the presence of formaldehyde according to the Mannich reaction to synthesize 1-(morpholine-4-yl-methyl)-3-alkyl(aryl)-4-(3-methoxy-4-isobutyryloxy-benzylideneamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones 5, and 1-(1-methylpiperazin-4-yl-methyl)-3-alkyl(aryl)-4-(3-methoxy-4-isobutyryloxybenzylideneamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones 6, respectively. The structures of twenty four new compounds have been characterized by IR, 1H and 13C NMR and MS spectroscopic methods. In addition, the newly synthesized compounds have been screened for their antimicrobial activities. Furthermore, these twenty four new compounds have been analyzed for their in vitro potential antioxidant activities by three different methods
Predictions for PP-wave string amplitudes from perturbative SYM
The role of general two-impurity multi-trace operators in the BMN
correspondence is explored. Surprisingly, the anomalous dimensions of all
two-impurity multi-trace BMN operators to order g_2^2\lambda' are completely
determined in terms of single-trace anomalous dimensions. This is due to
suppression of connected field theory diagrams in the BMN limit and this fact
has important implications for some string theory processes on the PP-wave
background. We also make gauge theory predictions for the matrix elements of
the light-cone string field theory Hamiltonian in the two string-two string and
one string-three string sectors.Comment: 46 pages, 12 figures. V3:typos correcte
Black-hole quasinormal modes and scalar glueballs in a finite-temperature AdS/QCD model
We use the holographic AdS/QCD soft-wall model to investigate the spectrum of
scalar glueballs in a finite temperature plasma. In this model, glueballs are
described by a massless scalar field in an AdS_5 black hole with a dilaton
soft-wall background. Using AdS/CFT prescriptions, we compute the boundary
retarded Green's function. The corresponding thermal spectral function shows
quasiparticle peaks at low temperatures. We also compute the quasinormal modes
of the scalar field in the soft-wall black hole geometry. The temperature and
momentum dependences of these modes are analyzed. The positions and widths of
the peaks of the spectral function are related to the frequencies of the
quasinormal modes. Our numerical results are found employing the power series
method and the computation of Breit-Wigner resonances.Comment: Revision: Results unchanged. More discussions on the model and on the
results. References added. 28 pages, 7 figures, 5 table
Robustness of Sound Speed and Jet Quenching for Gauge/Gravity Models of Hot QCD
We probe the effectiveness and robustness of a simple gauge/gravity dual
model of the QCD fireball that breaks conformal symmetry by constructing a
family of similar geometries that solve the scalar/gravity equations of motion.
This family has two parameters, one of which is associated to the temperature.
We calculate two quantities, the speed of sound and the jet-quenching
parameter. We find the speed of sound to be universal and robust over all the
geometries when appropriate units are used, while the jet-quenching parameter
varies significantly away from the conformal limit. We note that the overall
structure of the jet-quenching depends strongly on whether the running scalar
is the dilaton or not. We also discuss the variation of the scalar potential
over our family of solutions, and truncate our results to where the associated
error is small.Comment: 21 pages, 9 figures, LaTeX. v2:references added, minor correction to
speed of sound; conclusions unchange
Instability and Degeneracy in the BMN Correspondence
Non-degenerate perturbation theory, which was used to calculate the scale
dimension of operators on the gauge theory side of the correspondence, breaks
down when effects of triple trace operators are included. We interpret this as
an instability of excited single-string states in the dual string theory for
decay into the continuum of degenerate 3-string states. We apply time-dependent
perturbation theory to calculate the decay widths from gauge theory. These
widths are new gauge theory data which can be compared with future calculations
in light cone string field theory.Comment: 23 pages, no figure
Zero Sound in Effective Holographic Theories
We investigate zero sound in -dimensional effective holographic theories,
whose action is given by Einstein-Maxwell-Dilaton terms. The bulk spacetimes
include both zero temperature backgrounds with anisotropic scaling symmetry and
their near-extremal counterparts obtained in 1006.2124 [hep-th], while the
massless charge carriers are described by probe D-branes. We discuss
thermodynamics of the probe D-branes analytically. In particular, we clarify
the conditions under which the specific heat is linear in the temperature,
which is a characteristic feature of Fermi liquids. We also compute the
retarded Green's functions in the limit of low frequency and low momentum and
find quasi-particle excitations in certain regime of the parameters. The
retarded Green's functions are plotted at specific values of parameters in
, where the specific heat is linear in the temperature and the
quasi-particle excitation exists. We also calculate the AC conductivity in
-dimensions as a by-product.Comment: 29 pages, 1 figur
Thermodynamics of SU(N) Yang-Mills theories in 2+1 dimensions II - The deconfined phase
We present a non-perturbative study of the equation of state in the
deconfined phase of Yang-Mills theories in D=2+1 dimensions. We introduce a
holographic model, based on the improved holographic QCD model, from which we
derive a non-trivial relation between the order of the deconfinement phase
transition and the behavior of the trace of the energy-momentum tensor as a
function of the temperature T. We compare the theoretical predictions of this
holographic model with a new set of high-precision numerical results from
lattice simulations of SU(N) theories with N=2, 3, 4, 5 and 6 colors. The
latter reveal that, similarly to the D=3+1 case, the bulk equilibrium
thermodynamic quantities (pressure, trace of the energy-momentum tensor, energy
density and entropy density) exhibit nearly perfect proportionality to the
number of gluons, and can be successfully compared with the holographic
predictions in a broad range of temperatures. Finally, we also show that, again
similarly to the D=3+1 case, the trace of the energy-momentum tensor appears to
be proportional to T^2 in a wide temperature range, starting from approximately
1.2 T_c, where T_c denotes the critical deconfinement temperature.Comment: 2+36 pages, 10 figures; v2: comments added, curves showing the
holographic predictions included in the plots of the pressure and energy and
entropy densities, typos corrected: version published in JHE
Probes on D3-D7 Quark-Gluon Plasmas
We study the holographic dual model of quenched flavors immersed in a
quark-gluon plasma with massless dynamical quarks in the Veneziano limit. This
is modeled by embedding a probe D7 brane in a background where the backreaction
of massless D7 branes has been taken into account. The background, and hence
the effects, are perturbative in the Veneziano parameter N_f/N_c, therefore
giving small shifts of all magnitudes like the constituent mass, the quark
condensate, and several transport coefficients. We provide qualitative results
for the effect of flavor degrees of freedom on the probes. For example, the
meson melting temperature is enhanced, while the screening length is
diminished. The drag force is also enhanced.Comment: 31 pages, 17 figure
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