32,592 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 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 will be presented.Comment: 4 pages, 6 eps figures, to appear in the proceedings of Quark Matter
2011, 23-28 May 2011, Annecy, Franc
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
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 with exponent , which is larger than expected from relativistic
scatterings. Using the approach of Zakharov, L'vov and Falkovich, we analyse
possible Kolmogorov coefficients for relativistic -particle
processes, which give at most perturbatively for an energy cascade.
We discuss nonperturbative scenarios which lead to larger values. As an extreme
limit we find the result generically in an inherently nonperturbative
effective field theory situation, which coincides with results obtained by
Berges et al.\ in large- 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 .Comment: published versio
Generalized Boltzmann equations for on-shell particle production in a hot plasma
A novel refinement of the conventional treatment of Kadanoff--Baym equations
is suggested. Besides the Boltzmann equation another differential equation is
used for calculating the evolution of the non-equilibrium two-point function.
Although it was usually interpreted as a constraint on the solution of the
Boltzmann equation, we argue that its dynamics is relevant to the determination
and resummation of the particle production cut contributions. The differential
equation for this new contribution is illustrated in the example of the cubic
scalar model. The analogue of the relaxation time approximation is suggested.
It results in the shift of the threshold location and in smearing out of the
non-analytic threshold behaviour of the spectral function. Possible
consequences for the dilepton production are discussed.Comment: 22 pages, latex, 2 ps figure
The Abelianization of QCD Plasma Instabilities
QCD plasma instabilities appear to play an important role in the
equilibration of quark-gluon plasmas in heavy-ion collisions in the theoretical
limit of weak coupling (i.e. asymptotically high energy). It is important to
understand what non-linear physics eventually stops the exponential growth of
unstable modes. It is already known that the initial growth of plasma
instabilities in QCD closely parallels that in QED. However, once the unstable
modes of the gauge-fields grow large enough for non-Abelian interactions
between them to become important, one might guess that the dynamics of QCD
plasma instabilities and QED plasma instabilities become very different. In
this paper, we give suggestive arguments that non-Abelian self-interactions
between the unstable modes are ineffective at stopping instability growth, and
that the growing non-Abelian gauge fields become approximately Abelian after a
certain stage in their growth. This in turn suggests that understanding the
development of QCD plasma instabilities in the non-linear regime may have close
parallels to similar processes in traditional plasma physics. We conjecture
that the physics of collisionless plasma instabilities in SU(2) and SU(3) gauge
theory becomes equivalent, respectively, to (i) traditional plasma physics,
which is U(1) gauge theory, and (ii) plasma physics of U(1)x U(1) gauge theory.Comment: 36 pages; 15 figures [minor changes made to text, and new figure
added, to reflect published version
An Exact Universal Gravitational Lensing Equation
We first define what we mean by gravitational lensing equations in a general
space-time. A set of exact relations are then derived that can be used as the
gravitational lens equations in all physical situations. The caveat is that
into these equations there must be inserted a function, a two-parameter family
of solutions to the eikonal equation, not easily obtained, that codes all the
relevant (conformal) space-time information for this lens equation
construction. Knowledge of this two-parameter family of solutions replaces
knowledge of the solutions to the geodesic equations.
The formalism is then applied to the Schwarzschild lensing problemComment: 12 pages, submitted to Phys. Rev.
Centrality dependence of elliptic flow and QGP viscosity
In the Israel-Stewart's theory of second order hydrodynamics, we have
analysed the recent PHENIX data on charged particles elliptic flow in Au+Au
collisions.
PHENIX data demand more viscous fluid in peripheral collisions than in
central collisions. Over a broad range of collision centrality (0-10%- 50-60%),
viscosity to entropy ratio () varies between 0-0.17.Comment: Final version to be publiashed in J. Phys. G. 8 pages, 6 figures and
3 table
Uranium(III) coordination chemistry and oxidation in a flexible small-cavity macrocycle
U(III) complexes of the conformationally flexible, small-cavity macrocycle trans-calix[2]benzene[2]pyrrolide (L)2–, [U(L)X] (X = O-2,6-tBu2C6H3, N(SiMe3)2), have been synthesized from [U(L)BH4] and structurally characterized. These complexes show binding of the U(III) center in the bis(arene) pocket of the macrocycle, which flexes to accommodate the increase in the steric bulk of X, resulting in long U–X bonds to the ancillary ligands. Oxidation to the cationic U(IV) complex [U(L)X][B(C6F5)4] (X = BH4) results in ligand rearrangement to bind the smaller, harder cation in the bis(pyrrolide) pocket, in a conformation that has not been previously observed for (L)2–, with X located between the two ligand arene rings
Angular intricacies in hot gauge field theories
It is argued that in hot gauge field theories, "Hard Thermal Loops" leading
order calculations call for a definite sequence of angular averages and
discontinuity (or Imaginary part prescription) operations, and run otherwise
into incorrect results. The ten years old collinear singularity problem of hot
, provides a striking illustration of that fate.Comment: 14 pages, 1 figur
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