40,650 research outputs found

### Electrical conductivity and thermal dilepton rate from quenched lattice QCD

We report on a continuum extrapolation of the vector current correlation
function for light valence quarks in the deconfined phase of quenched QCD. This
is achieved by performing a systematic analysis of the influence of cut-off
effects on light quark meson correlators at $T\simeq 1.45 T_c$ using clover
improved Wilson fermions. We discuss resulting constraints on the electrical
conductivity and the thermal dilepton rate in a quark gluon plasma. In addition
new results at 1.2 and 3.0 $T_c$ will be presented.Comment: 4 pages, 6 eps figures, to appear in the proceedings of Quark Matter
2011, 23-28 May 2011, Annecy, Franc

### Effective Kinetic Theory for High Temperature Gauge Theories

Quasiparticle dynamics in relativistic plasmas associated with hot,
weakly-coupled gauge theories (such as QCD at asymptotically high temperature
$T$) can be described by an effective kinetic theory, valid on sufficiently
large time and distance scales. The appropriate Boltzmann equations depend on
effective scattering rates for various types of collisions that can occur in
the plasma. The resulting effective kinetic theory may be used to evaluate
observables which are dominantly sensitive to the dynamics of typical
ultrarelativistic excitations. This includes transport coefficients
(viscosities and diffusion constants) and energy loss rates. We show how to
formulate effective Boltzmann equations which will be adequate to compute such
observables to leading order in the running coupling $g(T)$ of high-temperature
gauge theories [and all orders in $1/\log g(T)^{-1}$]. As previously proposed
in the literature, a leading-order treatment requires including both $22$
particle scattering processes as well as effective ``$12$'' collinear
splitting processes in the Boltzmann equations. The latter account for nearly
collinear bremsstrahlung and pair production/annihilation processes which take
place in the presence of fluctuations in the background gauge field. Our
effective kinetic theory is applicable not only to near-equilibrium systems
(relevant for the calculation of transport coefficients), but also to highly
non-equilibrium situations, provided some simple conditions on distribution
functions are satisfied.Comment: 40 pages, new subsection on soft gauge field instabilities adde

### Real-time Chern-Simons term for hypermagnetic fields

If non-vanishing chemical potentials are assigned to chiral fermions, then a
Chern-Simons term is induced for the corresponding gauge fields. In thermal
equilibrium anomalous processes adjust the chemical potentials such that the
coefficient of the Chern-Simons term vanishes, but it has been argued that
there are non-equilibrium epochs in cosmology where this is not the case and
that, consequently, certain fermionic number densities and large-scale
(hypermagnetic) field strengths get coupled to each other. We generalise the
Chern-Simons term to a real-time situation relevant for dynamical
considerations, by deriving the anomalous Hard Thermal Loop effective action
for the hypermagnetic fields, write down the corresponding equations of motion,
and discuss some exponentially growing solutions thereof.Comment: 13 page

### Non-abelian plasma instabilities for strong anisotropy

We numerically investigate gauge field instabilities in anisotropic SU(2)
plasmas using weak field initial conditions. The growth of unstable modes is
stopped by non-abelian effects for moderate anisotropy. If we increase the
anisotropy the growth continues beyond the non-abelian saturation bound. We
find strong indications that the continued growth is not due to over-saturation
of infrared field modes, but instead due to very rapid growth of high momentum
modes which are not unstable in the weak field limit. The saturation amplitude
strongly depends on the initial conditions. For strong initial fields we do not
observe the sustained growth.Comment: 28 pages, 17 figure

### Hard thermal loops in the real-time formalism

We present a systematic discussion of Braaten and Pisarski's hard thermal
loop (HTL) effective theory within the framework of the real-time
(Schwinger-Keldysh) formalism. As is well known, the standard imaginary-time
HTL amplitudes for hot gauge theory express the polarization of a medium made
out of nonabelian charged point-particles; we show that the complete real-time
HTL theory includes, in addition, a second set of amplitudes which account for
Gaussian fluctuations in the charge distributions, but nothing else. We give a
concise set of graphical rules which generate both set of functions, and
discuss its relation to classical plasma physics.Comment: 14 pages, 6 figure

### The Three-Loop Free Energy for High-Temperature QED and QCD with Fermions

We compute the free energy density for gauge theories, with fermions, at high
temperature and zero chemical potential. Specifically, we analytically compute
the free energy through $O(g^4)$, which requires the evaluation of three-loop
diagrams. This computation extends our previous result for pure gauge QCD.Comment: 26 pages, 9 postscript figures, UW/PT-94-1

### High momentum lepton pairs from jet-plasma interactions

We discuss the emission of high momentum lepton pairs (p_T>4 GeV) with low
invariant masses (M << p_T) in central Au+Au collisions at RHIC
(\sqrt{s_{NN}}=200 GeV). The spectra of dileptons produced through interactions
of quark and antiquark jets with the quark-gluon plasma (QGP) have been
calculated. Annihilation and Compton scattering processes, as well as processes
benefitting from collinear enhancement, including Landau-Pomeranchuk-Migdal
(LPM) effects, are calculated and convolved with a one dimensional hydrodynamic
expansion. The jet-induced contributions are compared to thermal dilepton
emission and Drell-Yan processes, and are found to dominate around p_T=4 GeV.Comment: Parallel talk given at QM2006, Shanghai November 2006. 4 pages, 3
figure

### Perturbative and Nonperturbative Kolmogorov Turbulence in a Gluon Plasma

In numerical simulations of nonabelian plasma instabilities in the hard-loop
approximation, a turbulent spectrum has been observed that is characterized by
a phase-space density of particles $n(p)\sim p^{-\nu}$ with exponent $\nu\simeq
2$, which is larger than expected from relativistic $2\leftrightarrow 2$
scatterings. Using the approach of Zakharov, L'vov and Falkovich, we analyse
possible Kolmogorov coefficients for relativistic $(m \ge 4)$-particle
processes, which give at most $\nu=5/3$ perturbatively for an energy cascade.
We discuss nonperturbative scenarios which lead to larger values. As an extreme
limit we find the result $\nu=5$ generically in an inherently nonperturbative
effective field theory situation, which coincides with results obtained by
Berges et al.\ in large-$N$ scalar field theory. If we instead assume that
scaling behavior is determined by Schwinger-Dyson resummations such that the
different scaling of bare and dressed vertices matters, we find that
intermediate values are possible. We present one simple scenario which would
single out $\nu=2$.Comment: published versio

### A transport coefficient: the electrical conductivity

I describe the lattice determination of the electrical conductivity of the
quark gluon plasma. Since this is the first extraction of a transport
coefficient with a degree of control over errors, I next use this to make
estimates of other transport related quantities using simple kinetic theory
formulae. The resulting estimates are applied to fluctuations, ultra-soft
photon spectra and the viscosity. Dimming of ultra-soft photons is exponential
in the mean free path, and hence is a very sensitive probe of transport.Comment: Talk given in ICPAQGP 2005, SINP, Kolkat

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