1,240 research outputs found
Soft Photon Production Rate in Resummed Perturbation Theory of High Temperature QCD
We calculate the production rate of soft real photons from a hot quark --
gluon plasma using Braaten -- Pisarski's perturbative resummation method. To
leading order in the QCD coupling constant we find a logarithmically
divergent result for photon energies of order , where is the plasma
temperature. This divergent behaviour is due to unscreened mass singularities
in the effective hard thermal loop vertices in the case of a massless external
photon.Comment: 13 pages (2 figures not included), PLAINTEX, LPTHE-Orsay 93/46, BI-TP
93/5
THERMAL EFFECTS ON THE CATALYSIS BY A MAGNETIC FIELD
We show that the formation of condensates in the presence of a constant
magnetic field in 2+1 dimensions is extremely unstable. It disappears as soon
as a heat bath is introduced with or without a chemical potential. We point out
some new nonanalytic behavior that develops in this system at finite
temperature.Comment: 10 pages, plain Te
Low-momentum Pion Enhancement Induced by Chiral Symmetry Restoration
The thermal and nonthermal pion production by sigma decay and its relation
with chiral symmetry restoration in a hot and dense matter are investigated.
The nonthermal decay into pions of sigma mesons which are popularly produced in
chiral symmetric phase leads to a low-momentum pion enhancement as a possible
signature of chiral phase transition at finite temperature and density.Comment: 3 pages, 2 figure
On the Deconfinement Phase Transition in the Resonance Gas
We obtain the constraints on the ruling parameters of the dense hadronic gas
model at the critical temperature and propose the quasiuniversal ratios of the
thermodynamic quantities. The possible appearence of thermodynamical
instability in such a model is discussed.Comment: 7 pages, plain LaTeX, BI-TP 94/4
How fast can the wall move? A study of the electroweak phase transition dynamics
We consider the dynamics of bubble growth in the Minimal Standard Model at
the electroweak phase transition and determine the shape and the velocity of
the phase boundary, or bubble wall. We show that in the semi-classical
approximation the friction on the wall arises from the deviation of massive
particle populations from thermal equilibrium. We treat these with Boltzmann
equations in a fluid approximation. This approximation is reasonable for the
top quarks and the light species while it underestimates the friction from the
infrared bosons and Higgs particles. We use the two-loop finite temperature
effective potential and find a subsonic bubble wall for the whole range of
Higgs masses GeV. The result is weakly dependent on : the wall
velocity falls in the range , while the wall thickness is
in the range . The wall is thicker than the phase equilibrium
value because out of equilibrium particles exert more friction on the back than
on the base of a moving wall. We also consider the effect of an infrared gauge
condensate which may exist in the symmetric phase; modelling it simplemindedly,
we find that the wall may become supersonic, but not ultrarelativistic.Comment: 42 pages, plain latex, with three figures. Minor editing August 1 (we
figured out how to do analytically some integrals we previously did
numerically, made corresponding (slight) changes to numerical results, and
corrected some typos.
Thin-film flow in helically wound rectangular channels with small torsion
Laminar gravity-driven thin-film flow down a helically-wound channel of rectangular cross-section with small torsion in which the fluid depth is small is considered. Neglecting the entrance and exit regions we obtain the steady-state solution that is independent of position along the axis of the channel, so that the flow, which comprises a primary flow in the direction of the axis of the channel and a secondary flow in the cross-sectional plane, depends only on position in the two-dimensional cross-section of the channel. A thin-film approximation yields explicit expressions for the fluid velocity and pressure in terms of the free-surface shape, the latter satisfying a non-linear ordinary differential equation that has a simple exact solution in the special case of a channel of rectangular cross-section. The predictions of the thin-film model are shown to be in good agreement with much more computationally intensive solutions of the small-helix-torsion Navier–Stokes equations. The present work has particular relevance to spiral particle separators used in the mineral-processing industry. The validity of an assumption commonly used in modelling flow in spiral separators, namely that the flow in the outer region of the separator cross-section is described by a free vortex, is shown to depend on the problem parameters
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]
Excess electron pairs from heavy-ion collisions at CERN and a more complete picture of thermal production
The low-mass dielectron signal from heavy-ion collisions at the CERN--SPS
reported by the {\em CERES} collaboration is in excess of estimated hadronic
decays suggestive of possible contribution from two-pion annihilation or other
hadronic reactions. In the absence of dramatic medium modifications,
annihilation alone is unable to account for the data. We explore the role of
pion plus resonance scattering [] which
has favorable kinematics to populate masses between and .
While it seems to account for some of the remaining excess beyond annihilation,
it fails to allow quantitative interpretation of data.Comment: 11 pages LaTeX, 3 PostScript figure
Thermal and Chemical Equilibration in Relativistic Heavy Ion Collisions
We investigate the thermalization and the chemical equilibration of a parton
plasma created from Au+Au collision at LHC and RHIC energies starting from the
early moment when the particle momentum distributions in the central region
become for the first time isotropic due to longitudinal cooling. Using the
relaxation time approximation for the collision terms in the Boltzmann
equations for gluons and for quarks and the real collision terms constructed
from the simplest QCD interactions, we show that the collision times have the
right behaviour for equilibration. The magnitude of the quark (antiquark)
collision time remains bigger than the gluon collision time throughout the
lifetime of the plasma so that gluons are equilibrating faster than quarks both
chemically and kinetically. That is we have a two-stage equilibration scenario
as has been pointed out already by Shuryak sometimes ago. Full kinetic
equilibration is however slow and chemical equilibration cannot be completed
before the onset of the deconfinement phase transition assumed to be at
MeV. By comparing the collision entropy density rates of the
different processes, we show explicitly that inelastic processes, and
\emph{not} elastic processes as is commonly assumed, are dominant in the
equilibration of the plasma and that gluon branching leads the other processes
in entropy generation. We also show that, within perturbative QCD, processes
with higher power in \alpha_s need not be less important for the purpose of
equilibration than those with lower power. The state of equilibration of the
system has also a role to play. We compare our results with those of the parton
cascade model.Comment: 17 pages, revtex+psfig style with 14 embedded postscript figures, to
appear in Phys. Rev.
Two mechanisms for the elimination of pinch singularities in out of equilibrium thermal field theories
We analyze ill-defined pinch singularities characteristic of out of
equilibrium thermal field theories. We identify two mechanisms that eliminate
pinching even at the single self-energy insertion approximation to the
propagator: the first is based on the vanishing of phase space at the singular
point (threshold effect). It is effective in QED with a massive electron and a
massless photon. In massless QCD, this mechanism fails, but the pinches cancel
owing to the second mechanism, i.e., owing to the spinor/tensor structure of
the single self-energy insertion contribution to the propagator. The
constraints imposed on distribution functions are very reasonable.Comment: 24 pages, Latex, no figures, revised version, many minor changes and
correction
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