10,206 research outputs found
Gauge Invariance of Resummation Schemes: The QCD Partition Function
We pick up a method originally developed by Cheng and Tsai for vacuum
perturbation theory which allows to test the consistency of different sets of
Feynman rules on a purely diagrammatic level, making explicit loop calculations
superfluous. We generalize it to perturbative calculations in thermal field
theory and we show that it can be adapted to check the gauge invariance of
physical quantities calculated in improved perturbation schemes. Specifically,
we extend this diagrammatic technique to a simple resummation scheme in
imaginary time perturbation theory. As an application, we check up to O(g^4) in
general covariant gauge the gauge invariance of the result for the QCD
partition function which was recently obtained in Feynman gauge.Comment: 29 pages, LaTeX, using RevTeX and feynmf.sty, Replacement: NO changes
to the paper, TeX-source now additionally avaibl
High temperature color conductivity at next-to-leading log order
The non-Abelian analog of electrical conductivity at high temperature has
previously been known only at leading logarithmic order: that is, neglecting
effects suppressed only by an inverse logarithm of the gauge coupling. We
calculate the first sub-leading correction. This has immediate application to
improving, to next-to-leading log order, both effective theories of
non-perturbative color dynamics, and calculations of the hot electroweak baryon
number violation rate.Comment: 47 pages, 6+2 figure
Thermalization vs. Isotropization & Azimuthal Fluctuations
Hydrodynamic description requires a local thermodynamic equilibrium of the
system under study but an approximate hydrodynamic behaviour is already
manifested when a momentum distribution of liquid components is not of
equilibrium form but merely isotropic. While the process of equilibration is
relatively slow, the parton system becomes isotropic rather fast due to the
plasma instabilities. Azimuthal fluctuations observed in relativistic heavy-ion
collisions are argued to distinguish between a fully equilibrated and only
isotropic parton system produced in the collision early stage.Comment: 12 pages, presented at `Correlations and Fluctuations in Relativistic
Nuclear Collisions', MIT, April 05, minor correction
Fluctuations from dissipation in a hot non-Abelian plasma
We consider a transport equation of the Boltzmann-Langevin type for
non-Abelian plasmas close to equilibrium to derive the spectral functions of
the underlying microscopic fluctuations from the entropy. The correlator of the
stochastic source is obtained from the dissipative processes in the plasma.
This approach, based on classical transport theory, exploits the well-known
link between a linearized collision integral, the entropy and the spectral
functions. Applied to the ultra-soft modes of a hot non-Abelian (classical or
quantum) plasma, the resulting spectral functions agree with earlier findings
obtained from the microscopic theory. As a by-product, it follows that
B\"odeker's effective theory is consistent with the fluctuation-dissipation
theorem.Comment: 9 pages, revtex, no figures, identical to published versio
Whitening of the Quark-Gluon Plasma
Parton-parton collisions do not neutralize local color charges in the
quark-gluon plasma as they only redistribute the charges among momentum modes.
We discuss color diffusion and color conductivity as the processes responsible
for the neutralization of the plasma. For this purpose, we first compute the
conductivity and diffusion coefficients in the plasma that is significantly
colorful. Then, the time evolution of the color density due to the conductivity
and diffusion is studied. The conductivity is shown to be much more efficient
than the diffusion in neutralizing the plasma at the scale longer than the
screening length. Estimates of the characteristic time scales, which are based
on close to global equilibrium computations, suggest that first the plasma
becomes white and then the momentum degrees of freedom thermalize.Comment: 9 pages, revised, to appear in Phys. Rev.
Spin-Correlation Coefficients and Phase-Shift Analysis for p+He Elastic Scattering
Angular Distributions for the target spin-dependent observables A,
A, and A have been measured using polarized proton beams at
several energies between 2 and 6 MeV and a spin-exchange optical pumping
polarized He target. These measurements have been included in a global
phase-shift analysis following that of George and Knutson, who reported two
best-fit phase-shift solutions to the previous global p+He elastic
scattering database below 12 MeV. These new measurements, along with
measurements of cross-section and beam-analyzing power made over a similar
energy range by Fisher \textit{et al.}, allowed a single, unique solution to be
obtained. The new measurements and phase-shifts are compared with theoretical
calculations using realistic nucleon-nucleon potential models.Comment: Submitted to Phys. Rev.
Viscosity of High Energy Nuclear Fluids
Relativistic high energy heavy ion collision cross sections have been
interpreted in terms of almost ideal liquid droplets of nuclear matter. The
experimental low viscosity of these nuclear fluids have been of considerable
recent quantum chromodynamic interest. The viscosity is here discussed in terms
of the string fragmentation models wherein the temperature dependence of the
nuclear fluid viscosity obeys the Vogel-Fulcher-Tammann law.Comment: 6 pages, ReVTeX 4 format, two figures, *.eps forma
Dynamics of Quark-Gluon-Plasma Instabilities in Discretized Hard-Loop Approximation
Non-Abelian plasma instabilities have been proposed as a possible explanation
for fast isotropization of the quark-gluon plasma produced in relativistic
heavy-ion collisions. We study the real-time evolution of these instabilities
in non-Abelian plasmas with a momentum-space anisotropy using a hard-loop
effective theory that is discretized in the velocities of hard particles. We
extend our previous results on the evolution of the most unstable modes, which
are constant in directions transverse to the direction of anisotropy, from
gauge group SU(2) to SU(3). We also present first full 3+1-dimensional
simulation results based on velocity-discretized hard loops. In contrast to the
effectively 1+1-dimensional transversely constant modes we find subexponential
behaviour at late times.Comment: 30 pages, 16 figures. v3 typos fixe
Singularities and the distribution of density in the Burgers/adhesion model
We are interested in the tail behavior of the pdf of mass density within the
one and -dimensional Burgers/adhesion model used, e.g., to model the
formation of large-scale structures in the Universe after baryon-photon
decoupling. We show that large densities are localized near ``kurtoparabolic''
singularities residing on space-time manifolds of codimension two ()
or higher (). For smooth initial conditions, such singularities are
obtained from the convex hull of the Lagrangian potential (the initial velocity
potential minus a parabolic term). The singularities contribute {\em
\hbox{universal} power-law tails} to the density pdf when the initial
conditions are random. In one dimension the singularities are preshocks
(nascent shocks), whereas in two and three dimensions they persist in time and
correspond to boundaries of shocks; in all cases the corresponding density pdf
has the exponent -7/2, originally proposed by E, Khanin, Mazel and Sinai (1997
Phys. Rev. Lett. 78, 1904) for the pdf of velocity gradients in one-dimensional
forced Burgers turbulence. We also briefly consider models permitting particle
crossings and thus multi-stream solutions, such as the Zel'dovich approximation
and the (Jeans)--Vlasov--Poisson equation with single-stream initial data: they
have singularities of codimension one, yielding power-law tails with exponent
-3.Comment: LATEX 11 pages, 6 figures, revised; Physica D, in pres
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
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