1,856 research outputs found
Small shear viscosity in the semi quark gluon plasma
At nonzero temperature in QCD, about the deconfining phase transition there
is a "semi" quark gluon plasma (semi-QGP), where the expectation value of the
(renormalized) Polyakov loop is less than one. This can be modeled by a
semiclassical expansion about a constant field for the vector potential, A_0,
which is diagonal in color. We compute the shear viscosity in the semi-QGP by
using the Boltzmann equation in the presence of this background field. To
leading, logarithmic order in weak coupling, the dominant diagrams are given by
the usual scattering processes of 2 -> 2 particles. For simplicity we also
assume that both the number of colors and flavors are large. Near the critical
temperature T_c, where the expectation value of the Polyakov loop is small, the
overall density of colored fields decreases according to their color
representation, with the density of quarks vanishes linearly with the loop, and
that of gluons, quadratically. This decrease in the overall density dominates
changes in the transport cross section. As a result, relative to that in the
perturbative QGP, near T_c the shear viscosity in the semi-QGP is suppressed by
two powers of the Polyakov loop. In a semiclassical expansion, the suppression
of colored fields depends only upon which color representation they lie in, and
not upon their mass. That light and heavy quarks are suppressed in a common
manner may help to explain the behavior of charm quarks at RHIC.Comment: 45 pages, 8 figures, REVTeX; Abstract and Sec. III.A modified to
clarify the physical discussion
Zero Point Energy of Renormalized Wilson Loops
The quark antiquark potential, and its associated zero point energy, can be
extracted from lattice measurements of the Wilson loop. We discuss a unique
prescription to renormalize the Wilson loop, for which the perturbative
contribution to the zero point energy vanishes identically. A zero point energy
can arise nonperturbatively, which we illustrate by considering effective
string models. The nonperturbative contribution to the zero point energy
vanishes in the Nambu model, but is nonzero when terms for extrinsic curvature
are included. At one loop order, the nonperturbative contribution to the zero
point energy is negative, regardless of the sign of the extrinsic curvature
term.Comment: 14 pages, ReVTeX. Paper shortened, results unchange
Dilepton and photon production in the presence of a nontrivial Polyakov loop
We calculate the production of dileptons and photons in the presence of a
nontrivial Polyakov loop in QCD. This is applicable to the semi-Quark Gluon
Plasma (QGP), at temperatures above but near the critical temperature for
deconfinement. The Polyakov loop is small in the semi-QGP, and near unity in
the perturbative QGP. Working to leading order in the coupling constant of QCD,
we find that there is a mild enhancement, ~ 20%, for dilepton production in the
semi-QGP over that in the perturbative QGP. In contrast, we find that photon
production is strongly suppressed in the semi-QGP, by about an order of
magnitude, relative to the perturbative QGP. In the perturbative QGP photon
production contains contributions from 2->2 scattering and collinear emission
with the Landau- Pomeranchuk-Migdal (LPM) effect. In the semi-QGP we show that
the two contributions are modified differently. The rate for 2->2 scattering is
suppressed by a factor which depends upon the Polyakov loop. In contrast, in an
SU(N) gauge theory the collinear rate is suppressed by 1/N, so that the LPM
effect vanishes at infinite N. To leading order in the semi-QGP at large N, we
compute the rate from 2->2 scattering to the leading logarithmic order and the
collinear rate to leading order
Alpha-cluster structure and density wave in oblate nuclei
Pentagon and triangle shapes in Si-28 and C-12 are discussed in relation with
nuclear density wave. In the antisymmetrized molecular dynamics calculations,
the band in Si-28 and the band in C-12 are described by
the pentagon and triangle shapes, respectively. These negative-parity bands can
be interpreted as the parity partners of the ground bands and they
are constructed from the parity-asymmetric-intrinsic states. The pentagon and
the triangle shapes originate in 7alpha and 3alpha cluster structures,
respectively. In a mean-field picture, they are described also by the static
one-dimensional density wave at the edge of the oblate states. In analysis with
ideal alpha cluster models using Brink-Bloch cluster wave functions and that
with a simplified model, we show that the static edge density wave for the
pentagon and triangle shapes can be understood by spontaneous breaking of axial
symmetry, i.e., the instability of the oblate states with respect to the edge
density wave. The density wave is enhanced in the Z=N nuclei due to the
proton-neutron coherent density waves, while it is suppressed in Z\ne N nuclei.Comment: 23 pages, 8 figure
How Wide is the Transition to Deconfinement?
Pure SU(3) glue theories exhibit a deconfining phase transition at a nonzero
temperature, Tc. Using lattice measurements of the pressure, we develop a
simple matrix model to describe the transition region, when T > Tc. This model,
which involves three parameters, is used to compute the behavior of the 't
Hooft loop. There is a Higgs phase in this region, where off diagonal color
modes are heavy, and diagonal modes are light. Lattice measurements of the
latter suggests that the transition region is narrow, extending only to about
1.2 Tc. This is in stark contrast to lattice measurements of the renormalized
Polyakov loop, which indicates a much wider width. The possible implications
for the differences in heavy ion collisions between RHIC and the LHC are
discussed.Comment: v2: Minor changes in wording, references adde
Active Brownian Motion in Threshold Distribution of a Coulomb Blockade Model
Randomly-distributed offset charges affect the nonlinear current-voltage
property via the fluctuation of the threshold voltage of Coulomb blockade
arrays. We analytically derive the distribution of the threshold voltage for a
model of one-dimensional locally-coupled Coulomb blockade arrays, and propose a
general relationship between conductance and the distribution. In addition, we
show the distribution for a long array is equivalent to the distribution of the
number of upward steps for aligned objects of different height. The
distribution satisfies a novel Fokker-Planck equation corresponding to active
Brownian motion. The feature of the distribution is clarified by comparing it
with the Wigner and Ornstein-Uhlenbeck processes. It is not restricted to the
Coulomb blockade model, but instructive in statistical physics generally.Comment: 4pages, 3figure
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