931 research outputs found
A non-perturbative estimate of the heavy quark momentum diffusion coefficient
We estimate the momentum diffusion coefficient of a heavy quark within a pure
SU(3) plasma at a temperature of about 1.5Tc. Large-scale Monte Carlo
simulations on a series of lattices extending up to 192^3*48 permit us to carry
out a continuum extrapolation of the so-called colour-electric imaginary-time
correlator. The extrapolated correlator is analyzed with the help of
theoretically motivated models for the corresponding spectral function.
Evidence for a non-zero transport coefficient is found and, incorporating
systematic uncertainties reflecting model assumptions, we obtain kappa = (1.8 -
3.4)T^3. This implies that the "drag coefficient", characterizing the time
scale at which heavy quarks adjust to hydrodynamic flow, is (1.8 - 3.4)
(Tc/T)^2 (M/1.5GeV) fm/c, where M is the heavy quark kinetic mass. The results
apply to bottom and, with somewhat larger systematic uncertainties, to charm
quarks.Comment: 18 pages. v2: clarifications adde
Lattice constraints on the thermal photon rate
We estimate the photon production rate from an SU(3) plasma at temperatures
of about 1.1Tc and 1.3Tc. Lattice results for the vector current correlator at
spatial momenta k ~ (2-6)T are extrapolated to the continuum limit and analyzed
with the help of a polynomial interpolation for the corresponding spectral
function, which vanishes at zero frequency and matches to high-precision
perturbative results at large invariant masses. For small invariant masses the
interpolation is compared with the NLO weak-coupling result, hydrodynamics, and
a holographic model. At vanishing invariant mass we extract the photon rate
which for k \gsim 3T is found to be close to the NLO weak-coupling prediction.
For k \lsim 2T uncertainties remain large but the photon rate is likely to fall
below the NLO prediction, in accordance with the onset of a strongly
interacting behaviour characteristic of the hydrodynamic regime.Comment: 20 pages. v2: clarifications adde
Charge transport and vector meson dissociation across the thermal phase transition in lattice QCD with two light quark flavors
We compute and analyze correlation functions in the isovector vector channel
at vanishing spatial momentum across the deconfinement phase transition in
lattice QCD. The simulations are carried out at temperatures and with MeV for two flavors of Wilson-Clover
fermions with a zero-temperature pion mass of MeV. Exploiting exact
sum rules and applying a phenomenologically motivated ansatz allows us to
determine the spectral function via a fit to the lattice
correlation function data. From these results we estimate the electrical
conductivity across the deconfinement phase transition via a Kubo formula and
find evidence for the dissociation of the meson by resolving its
spectral weight at the available temperatures. We also apply the Backus-Gilbert
method as a model-independent approach to this problem. At any given frequency,
it yields a local weighted average of the true spectral function. We use this
method to compare kinetic theory predictions and previously published
phenomenological spectral functions to our lattice study.Comment: 28 pages, 6 figure
Massive vector current correlator in thermal QCD
We present an NLO analysis of the massive vector current correlator at
temperatures above a few hundred MeV. The physics of this correlator originates
from a transport peak, related to heavy quark diffusion, and from the
quark-antiquark threshold, related to quarkonium physics. In the bottom case
both can be studied with separate effective theories, but for charm these may
not be accurate, so a study within the full theory is needed. Working in
imaginary time, the NLO correlator can be computed in unresummed perturbation
theory; comparing with lattice data, we find good agreement. Subsequently we
inspect how non-perturbative modifications of the transport peak would affect
the imaginary-time correlator. The massive NLO quark-number susceptibility is
also contrasted with numerical measurement.Comment: 28 pages. v2: references and clarifications added, published versio
Quantitative photoacoustic imaging in radiative transport regime
The objective of quantitative photoacoustic tomography (QPAT) is to
reconstruct optical and thermodynamic properties of heterogeneous media from
data of absorbed energy distribution inside the media. There have been
extensive theoretical and computational studies on the inverse problem in QPAT,
however, mostly in the diffusive regime. We present in this work some numerical
reconstruction algorithms for multi-source QPAT in the radiative transport
regime with energy data collected at either single or multiple wavelengths. We
show that when the medium to be probed is non-scattering, explicit
reconstruction schemes can be derived to reconstruct the absorption and the
Gruneisen coefficients. When data at multiple wavelengths are utilized, we can
reconstruct simultaneously the absorption, scattering and Gruneisen
coefficients. We show by numerical simulations that the reconstructions are
stable.Comment: 40 pages, 13 figure
Transport properties and Langevin dynamics of heavy quarks and quarkonia in the Quark Gluon Plasma
Quark Gluon Plasma transport coefficients for heavy quarks and
quark-antiquark pairs are computed through an extension of the results obtained
for a hot QED plasma by describing the heavy-quark propagation in the eikonal
approximation and by weighting the gauge field configurations with the Hard
Thermal Loop effective action. It is shown that such a model allows to
correctly reproduce, at leading logarithmic accuracy, the results obtained by
other independent approaches. The results are then inserted into a relativistic
Langevin equation allowing to follow the evolution of the heavy-quark momentum
spectra. Our numerical findings are also compared with the ones obtained in a
strongly-coupled scenario, namely with the transport coefficients predicted
(though with some limitations and ambiguities) by the AdS/CFT correspondence.Comment: Minor changes. One figure added. Final version accepted for
publication by Nucl. Phys.
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