1,725 research outputs found
The Bulk Channel in Thermal Gauge Theories
We investigate the thermal correlator of the trace of the energy-momentum
tensor in the SU(3) Yang-Mills theory. Our goal is to constrain the spectral
function in that channel, whose low-frequency part determines the bulk
viscosity. We focus on the thermal modification of the spectral function,
. Using the operator-product expansion we give
the high-frequency behavior of this difference in terms of thermodynamic
potentials. We take into account the presence of an exact delta function
located at the origin, which had been missed in previous analyses. We then
combine the bulk sum rule and a Monte-Carlo evaluation of the Euclidean
correlator to determine the intervals of frequency where the spectral density
is enhanced or depleted by thermal effects. We find evidence that the thermal
spectral density is non-zero for frequencies below the scalar glueball mass
and is significantly depleted for .Comment: (1+25) pages, 6 figure
An elementary stringy estimate of transport coefficients of large temperature QCD
Modeling QCD at large temperature with a simple holographic five dimensional
theory encoding minimal breaking of conformality, allows for the calculation of
all the transport coefficients, up to second order, in terms of a single
parameter. In particular, the shear and bulk relaxation times are provided. The
result follows by deforming the AdS background with a scalar dual to a
marginally relevant operator, at leading order in the deformation parameter.Comment: 11 pages; v2: comments and references adde
Bulk spectral function sum rule in QCD-like theories with a holographic dual
We derive the sum rule for the spectral function of the stress-energy tensor
in the bulk (uniform dilatation) channel in a general class of strongly coupled
field theories. This class includes theories holographically dual to a theory
of gravity coupled to a single scalar field, representing the operator of the
scale anomaly. In the limit when the operator becomes marginal, the sum rule
coincides with that in QCD. Using the holographic model, we verify explicitly
the cancellation between large and small frequency contributions to the
spectral integral required to satisfy the sum rule in such QCD-like theories.Comment: 16 pages, 2 figure
The Lorentz-invariant boundary action of the confining string and its universal contribution to the inter-quark potential
We study the boundary contribution to the low energy effective action of the
open string describing the confining flux tube in gauge theories. The form of
the boundary terms is strongly constrained by the requirement of Lorentz
symmetry, which is spontaneously broken by the formation of a long confining
flux tube in the vacuum. Writing the boundary action as an expansion in the
derivatives of the Nambu-Goldstone modes describing the transverse fluctuations
of the string, we single out and put in a closed form the first few Lorentz
invariant boundary terms. We also evaluate the leading deviation from the
Nambu-Goto string produced by the boundary action on the vacuum expectation
value of the Wilson loop and we test this prediction in the 3d Ising gauge
model. Our simulation attains a level of precision which is sufficient to test
the contribution of this term.Comment: 17 pages, 5 figures, LateX 2e. V2: Final version published on JHEP.
Fixed typos in eq.s 2.2, 2.3, 3.7, 3.8, A.4. Extended explanation of the
procedures used in sec 2 to determine the possible boundary terms up to field
redefinitions and of the procedure used in sec 4 to take the continuum limit.
V3: typos corrected in eq.s (4.3) (4.5) and (4.6), acknowledgements adde
A Dynamic Model of Interactions of Ca^(2+), Calmodulin, and Catalytic Subunits of Ca^(2+)/Calmodulin-Dependent Protein Kinase II
During the acquisition of memories, influx of Ca^(2+) into the postsynaptic spine through the pores of activated N-methyl-D-aspartate-type glutamate receptors triggers processes that change the strength of excitatory synapses. The pattern of Ca^(2+) influx during the first few seconds of activity is interpreted within the Ca^(2+)-dependent signaling network such that synaptic strength is eventually either potentiated or depressed. Many of the critical signaling enzymes that control synaptic plasticity, including Ca^(2+)/calmodulin-dependent protein kinase II (CaMKII), are regulated by calmodulin, a small protein that can bind up to 4 Ca^(2+) ions. As a first step toward clarifying how the Ca^(2+)-signaling network decides between potentiation or depression, we have created a kinetic model of the interactions of Ca^(2+), calmodulin, and CaMKII that represents our best understanding of the dynamics of these interactions under conditions that resemble those in a postsynaptic spine. We constrained parameters of the model from data in the literature, or from our own measurements, and then predicted time courses of activation and autophosphorylation of CaMKII under a variety of conditions. Simulations showed that species of calmodulin with fewer than four bound Ca^(2+) play a significant role in activation of CaMKII in the physiological regime, supporting the notion that processing ofCa^(2+) signals in a spine involves competition among target enzymes for binding to unsaturated species of CaM in an environment in which the concentration of Ca^(2+) is fluctuating rapidly. Indeed, we showed that dependence of activation on the frequency of Ca^(2+) transients arises from the kinetics of interaction of fluctuating Ca^(2+) with calmodulin/CaMKII complexes. We used parameter sensitivity analysis to identify which parameters will be most beneficial to measure more carefully to improve the accuracy of predictions. This model provides a quantitative base from which to build more complex dynamic models of postsynaptic signal transduction during learning
Visual parameter optimisation for biomedical image processing
Background: Biomedical image processing methods require users to optimise input parameters to ensure high quality
output. This presents two challenges. First, it is difficult to optimise multiple input parameters for multiple
input images. Second, it is difficult to achieve an understanding of underlying algorithms, in particular, relationships
between input and output.
Results: We present a visualisation method that transforms users’ ability to understand algorithm behaviour by
integrating input and output, and by supporting exploration of their relationships. We discuss its application to a
colour deconvolution technique for stained histology images and show how it enabled a domain expert to
identify suitable parameter values for the deconvolution of two types of images, and metrics to quantify
deconvolution performance. It also enabled a breakthrough in understanding by invalidating an underlying
assumption about the algorithm.
Conclusions: The visualisation method presented here provides analysis capability for multiple inputs and outputs
in biomedical image processing that is not supported by previous analysis software. The analysis supported by our
method is not feasible with conventional trial-and-error approaches
The Sphaleron Rate in SU(N) Gauge Theory
The sphaleron rate is defined as the diffusion constant for topological
number NCS = int g^2 F Fdual/32 pi^2. It establishes the rate of equilibration
of axial light quark number in QCD and is of interest both in electroweak
baryogenesis and possibly in heavy ion collisions. We calculate the
weak-coupling behavior of the SU(3) sphaleron rate, as well as making the most
sensible extrapolation towards intermediate coupling which we can. We also
study the behavior of the sphaleron rate at weak coupling at large Nc.Comment: 18 pages with 3 figure
Hydrodynamics of fundamental matter
First and second order transport coefficients are calculated for the strongly
coupled N=4 SYM plasma coupled to massless fundamental matter in the Veneziano
limit. The results, including among others the value of the bulk viscosity and
some relaxation times, are presented at next-to-leading order in the flavor
contribution. The bulk viscosity is found to saturate Buchel's bound. This
result is also captured by an effective single-scalar five-dimensional
holographic dual in the Chamblin-Reall class and it is suggested to hold, in
the limit of small deformations, for generic plasmas with gravity duals,
whenever the leading conformality breaking effects are driven by marginally
(ir)relevant operators. This proposal is then extended to other relations for
hydrodynamic coefficients, which are conjectured to be universal for every
non-conformal plasma with a dual Chamblin-Reall-like description. Our analysis
extends to any strongly coupled gauge theory describing the low energy dynamics
of Nc>>1 D3-branes at the tip of a generic Calabi-Yau cone. The fundamental
fields are added by means of 1<<Nf<<Nc homogeneously smeared D7-branes.Comment: 24 pages. V2: Important improvements in the discussion of the results
in section 1. References adde
Thermal photons in QGP and non-ideal effects
We investigate the thermal photon production-rates using one dimensional
boost-invariant second order relativistic hydrodynamics to find proper time
evolution of the energy density and the temperature. The effect of
bulk-viscosity and non-ideal equation of state are taken into account in a
manner consistent with recent lattice QCD estimates. It is shown that the
\textit{non-ideal} gas equation of state i.e behaviour
of the expanding plasma, which is important near the phase-transition point,
can significantly slow down the hydrodynamic expansion and thereby increase the
photon production-rates. Inclusion of the bulk viscosity may also have similar
effect on the hydrodynamic evolution. However the effect of bulk viscosity is
shown to be significantly lower than the \textit{non-ideal} gas equation of
state. We also analyze the interesting phenomenon of bulk viscosity induced
cavitation making the hydrodynamical description invalid. We include the
viscous corrections to the distribution functions while calculating the photon
spectra. It is shown that ignoring the cavitation phenomenon can lead to
erroneous estimation of the photon flux.Comment: 11 pages, 13 figures; accepted for publication in JHE
Recommended from our members
What Google Maps can do for biomedical data dissemination: examples and a design study
BACKGROUND: Biologists often need to assess whether unfamiliar datasets warrant the time investment required for more detailed exploration. Basing such assessments on brief descriptions provided by data publishers is unwieldy for large datasets that contain insights dependent on specific scientific questions. Alternatively, using complex software systems for a preliminary analysis may be deemed as too time consuming in itself, especially for unfamiliar data types and formats. This may lead to wasted analysis time and discarding of potentially useful data.
RESULTS: We present an exploration of design opportunities that the Google Maps interface offers to biomedical data visualization. In particular, we focus on synergies between visualization techniques and Google Maps that facilitate the development of biological visualizations which have both low-overhead and sufficient expressivity to support the exploration of data at multiple scales. The methods we explore rely on displaying pre-rendered visualizations of biological data in browsers, with sparse yet powerful interactions, by using the Google Maps API. We structure our discussion around five visualizations: a gene co-regulation visualization, a heatmap viewer, a genome browser, a protein interaction network, and a planar visualization of white matter in the brain. Feedback from collaborative work with domain experts suggests that our Google Maps visualizations offer multiple, scale-dependent perspectives and can be particularly helpful for unfamiliar datasets due to their accessibility. We also find that users, particularly those less experienced with computer use, are attracted by the familiarity of the Google Maps API. Our five implementations introduce design elements that can benefit visualization developers.
CONCLUSIONS: We describe a low-overhead approach that lets biologists access readily analyzed views of unfamiliar scientific datasets. We rely on pre-computed visualizations prepared by data experts, accompanied by sparse and intuitive interactions, and distributed via the familiar Google Maps framework. Our contributions are an evaluation demonstrating the validity and opportunities of this approach, a set of design guidelines benefiting those wanting to create such visualizations, and five concrete example visualizations
- …