2,465 research outputs found
Ward Identities in Non-equilibrium QED
We verify the QED Ward identity for the two- and three -point functions at
non-equilibrium in the HTL limit. We use the Keldysh formalism of real time
finite temperature field theory. We obtain an identity of the same form as the
Ward identity for a set of one loop self-energy and one loop three-point vertex
diagrams which are constructed from HTL effective propagators and vertices.Comment: 19 pages, RevTex, 4 PostScript figures, revised version to be
published in Phys. Rev.
Autowaves in a dc complex plasma confined behind a de Laval nozzle
Experiments to explore stability conditions and topology of a dense
microparticle cloud supported against gravity by a gas flow were carried out.
By using a nozzle shaped glass insert within the glass tube of a dc discharge
plasma chamber a weakly ionized gas flow through a de Laval nozzle was
produced. The experiments were performed using neon gas at a pressure of 100 Pa
and melamine-formaldehyde particles with a diameter of 3.43 {\mu}m. The
capturing and stable global confining of the particles behind the nozzle in the
plasma were demonstrated. The particles inside the cloud behaved as a single
convection cell inhomogeneously structured along the nozzle axis in a tube-like
manner. The pulsed acceleration localized in the very head of the cloud
mediated by collective plasma-particle interactions and the resulting wave
pattern were studied in detail.Comment: 6 pages, 4 figure
Bremsstrahlung from an Equilibrating Quark-Gluon Plasma
The photon production rate from a chemically equilibrating quark-gluon plasma
likely to be produced at RHIC (BNL) and LHC (CERN) energies is estimated taking
into account bremsstrahlung. The plasma is assumed to be in local thermal
equilibrium, but with a phase space distribution that deviates from the Fermi
or Bose distribution by space-time dependent factors (fugacities). The photon
spectrum is obtained by integrating the photon rate over the space-time history
of the plasma, adopting a boost invariant cylindrically symmetric transverse
expansion of the system with different nuclear profile functions. Initial
conditions obtained from a self-screened parton cascade calculation and, for
comparison, from the HIJING model are used. Compared to an equilibrated plasma
at the same initial energy density, taken from the self-screened parton
cascade, a moderate suppression of the photon yield by a factor of one to five
depending on the collision energy and the photon momentum is observed. The
individual contributions to the photon production, however, are completely
different in the both scenarios.Comment: 14 pages, 4 figures, shortened version to be published in Phys. Rev.
Emittance growth in linear induction accelerators
The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung
radiation source spots produced by the focused electron beams from two linear
induction accelerators (LIAs) to radiograph large hydrodynamic experiments
driven by high explosives. Radiographic resolution is determined by the size of
the source spot, and beam emittance is the ultimate limitation to spot size. On
the DARHT Axis-II LIA we measure an emittance higher than predicted by
theoretical simulations, and even though this axis produces sub-millimeter
source spots, we are exploring ways to improve the emittance. Some of the
possible causes for the discrepancy have been investigated using
particle-in-cell (PIC) codes, although most of these are discounted based on
beam measurements. The most likely source of emittance growth is a mismatch of
the beam to the magnetic transport, which can cause beam halo.Comment: 20th Int. Conf. on High-Power Particle Beams, Washington, DC, May,
201
Parton Equilibration in Relativistic Heavy Ion Collisions
We investigate the processes leading to phase-space equilibration of parton
distributions in nuclear interactions at collider energies. We derive a set of
rate equations describing the chemical equilibration of gluons and quarks
including medium effects on the relevant QCD transport coefficients, and
discuss their consequences for parton equilibration in heavy ion collisions.Comment: 18 pages, 6 Figures appended as uuencoded PostScript files, (no
changes in the previously submitted manuscript), DUKE-TH-93-4
Expansion, Thermalization and Entropy Production in High-Energy Nuclear Collisions
The thermalization process is studied in an expanding parton gas using the
Boltzmann equation with two types of collision terms. In the relaxation time
approximation we determine the criteria under which a time-dependent relaxation
time leads to thermalization of the partons. We calculate the entropy
production due to collisions for the general time-dependent relaxation time. In
a perturbative QCD approach on the other hand, we can estimate the parton
collision time and its dependence on expansion time. The effective `out of
equilibrium' collision time differs from the standard transport relaxation
time, , by a weak time
dependence. It is in both cases Debye screening and Landau damping that
regulate the singular forward scattering processes. We find that the parton gas
does thermalize eventually but only after having undergone a phase of free
streaming and gradual equilibration where considerable entropy is produced
(``after-burning"). The final entropy and thus particle density depends on the
collision time as well as the initial conditions (a ``memory effect"). Results
for entropy production are presented based upon various model estimates of
early parton production.Comment: 15 pages revtex + 4 figures. Figures can be obtained by supplying
address to: [email protected]
Parton Interaction Rates in the Quark-Gluon Plasma
The transport interaction rates of elastic scattering processes of thermal
partons in the quark-gluon plasma are calculated beyond the leading logarithm
approximation using the effective perturbation theory for QCD at finite
temperatures developed by Braaten and Pisarski. The results for the ordinary
and transport interaction rates obtained from the effective perturbation theory
are compared to perturbative approximations based on an infrared cut-off by the
Debye screening mass. The relevance of those interaction rates for a
quark-gluon plasma possibly formed in ultrarelativistic heavy ion collisions
are discussed.Comment: 11 pages, 2 figures (not included), REVTex, UGI-93-0
Multiparticle production in the Glasma at NLO and plasma instabilities
We discuss the relation between multi-particle production in the Glasma at
next-to-leading order and the physics of plasma instabilities.Comment: 4 pages, talk at Quark Matter 200
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