3,698 research outputs found
Heavy quark medium polarization at next-to-leading order
We compute the imaginary part of the heavy quark contribution to the photon
polarization tensor, i.e. the quarkonium spectral function in the vector
channel, at next-to-leading order in thermal QCD. Matching our result, which is
valid sufficiently far away from the two-quark threshold, with a previously
determined resummed expression, which is valid close to the threshold, we
obtain a phenomenological estimate for the spectral function valid for all
non-zero energies. In particular, the new expression allows to fix the overall
normalization of the previous resummed one. Our result may be helpful for
lattice reconstructions of the spectral function (near the continuum limit),
which necessitate its high energy behaviour as input, and can in principle also
be compared with the dilepton production rate measured in heavy ion collision
experiments. In an appendix analogous results are given for the scalar channel.Comment: 43 pages. v2: a figure and other clarifications added, published
versio
A way to estimate the heavy quark thermalization rate from the lattice
The thermalization rate of a heavy quark is related to its momentum diffusion
coefficient. Starting from a Kubo relation and using the framework of the heavy
quark effective theory, we argue that in the large-mass limit the momentum
diffusion coefficient can be defined through a certain Euclidean correlation
function, involving color-electric fields along a Polyakov loop. Furthermore,
carrying out a perturbative computation, we demonstrate that the spectral
function corresponding to this correlator is relatively flat at small
frequencies. Therefore, unlike in the case of several other transport
coefficients, for which the narrowness of the transport peak makes analytic
continuation from Euclidean lattice data susceptible to severe systematic
uncertainties, it appears that the determination of the heavy quark
thermalization rate could be relatively well under control.Comment: 17 pages. v2: clarifications and references added, published versio
Application of a Linear Center Identification Scheme to Deterministic Polar Positioning
In a number of manufacturing applications, parts of circular cross-section must be centered for optimal processing or measurement. However, part form is never perfect, making accurate determination of the âcenteredâ state of a part difficult. Imperfect inputs to the manufacturing process such as rough-processed parts, deformation due to heat treatment, or raw formed materials present difficulty in centering by the traditional manual method. This paper presents a filtering and quantification technique for identifying the true center of an imperfect round part through isolation of the lowest polar frequency component. A low-cost device is presented that centers parts based on this frequency domain identification of center
The ultraviolet limit and sum rule for the shear correlator in hot Yang-Mills theory
We determine a next-to-leading order result for the correlator of the shear
stress operator in high-temperature Yang-Mills theory. The computation is
performed via an ultraviolet expansion, valid in the limit of small distances
or large momenta, and the result is used for writing operator product
expansions for the Euclidean momentum and coordinate space correlators as well
as for the Minkowskian spectral density. In addition, our results enable us to
confirm and refine a shear sum rule originally derived by Romatschke, Son and
Meyer.Comment: 16 pages, 2 figures. v2: small clarifications, one reference added,
published versio
A non-perturbative contribution to jet quenching
It has been argued by Caron-Huot that infrared contributions to the jet
quenching parameter in hot QCD, denoted by qhat, can be extracted from an
analysis of a certain static-potential related observable within the
dimensionally reduced effective field theory. Following this philosophy, the
order of magnitude of a non-perturbative contribution to qhat from the
colour-magnetic scale, g^2T/pi, is estimated. The result is small; it is
probably below the parametrically perturbative but in practice slowly
convergent contributions from the colour-electric scale, whose all-orders
resummation therefore remains an important challenge.Comment: 4 pages. v2: clarifications, published versio
Quark mass thresholds in QCD thermodynamics
We discuss radiative corrections to how quark mass thresholds are crossed, as
a function of the temperature, in basic thermodynamic observables such as the
pressure, the energy and entropy densities, and the heat capacity of high
temperature QCD. The indication from leading order that the charm quark plays a
visible role at surprisingly low temperatures, is confirmed. We also sketch a
way to obtain phenomenological estimates relevant for generic expansion rate
computations at temperatures between the QCD and electroweak scales, pointing
out where improvements over the current knowledge are particularly welcome.Comment: 14 pages. v2: minor additions and clarifications; published versio
Gauge Dependence of the High-Temperature 2-Loop Effective Potential for the Higgs Field
The high-temperature limit of the 2-loop effective potential for the Higgs
field is calculated from an effective 3d theory, in a general covariant gauge.
It is shown explicitly that a gauge-independent result can be extracted for the
equation of state from the gauge-dependent effective potential. The convergence
of perturbation theory is estimated in the broken phase, utilizing the gauge
dependence of the effective potential.Comment: 13 LaTeX-pages + 2 ps-figure (Instructions added to uudecode the
ps-file.
Bayesian Transductive Markov Random Fields for Interactive Segmentation in Retinal Disorders
In the realm of computer aided diagnosis (CAD) interactive segmentation schemes have been well received by physicians, where the combination of human and machine intelligence can provide improved segmentation efficacy with minimal expert intervention [1-3]. Transductive learning (TL) or semi-supervised learning (SSL) is a suitable framework for learning-based interactive segmentation given the scarce label problem. In this paper we present extended work on Bayesian transduction and regularized conditional mixtures for interactive segmentation [3]. We present a Markov random field model integrating a semi-parametric conditional mixture model within a Bayesian transductive learning and inference setting. The model allows efficient learning and inference in a semi-supervised setting given only minimal approximate label information. Preliminary experimental results on multimodal images of retinal disorders such as drusen, geographic atrophy (GA), and choroidal neovascularisation (CNV) with exudates and subretinal fibrosis show promising segmentation performance
Quarkonium in Hot Medium
I review recent progress in studying quarkonium properties in hot medium as
well as possible consequences for quarkonium production in heavy ion
collisions.Comment: Invited talk at SQM 2009, Buzios, Brazil, Sep. 27 -Oct. 2 2009,
LaTeX, 8 pages,3 figures; typos corrected, references adde
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