66,236 research outputs found
Fourier analysis of the flux-tube distribution in SU(3) lattice QCD
This letter presents a novel analysis of the action/energy density
distribution around a static quark-antiquark pair in SU(3) lattice quantum
chromodynamics. Using the Fourier transformation of the link variable, we
remove the high-momentum gluon and extract the flux-tube component from the
action/energy density. When the high-momentum gluon is removed, the statistical
fluctuation is drastically suppressed, and the singularities from the quark
self-energy disappear. The obtained flux-tube component is broadly distributed
around the line connecting the quark and the antiquark.Comment: 12 page
Clustering of vertically constrained passive particles in homogeneous, isotropic turbulence
We analyze the dynamics of small particles vertically confined, by means of a
linear restoring force, to move within a horizontal fluid slab in a
three-dimensional (3D) homogeneous isotropic turbulent velocity field. The
model that we introduce and study is possibly the simplest description for the
dynamics of small aquatic organisms that, due to swimming, active regulation of
their buoyancy, or any other mechanism, maintain themselves in a shallow
horizontal layer below the free surface of oceans or lakes. By varying the
strength of the restoring force, we are able to control the thickness of the
fluid slab in which the particles can move. This allows us to analyze the
statistical features of the system over a wide range of conditions going from a
fully 3D incompressible flow (corresponding to the case of no confinement) to
the extremely confined case corresponding to a two-dimensional slice. The
background 3D turbulent velocity field is evolved by means of fully resolved
direct numerical simulations. Whenever some level of vertical confinement is
present, the particle trajectories deviate from that of fluid tracers and the
particles experience an effectively compressible velocity field. Here, we have
quantified the compressibility, the preferential concentration of the
particles, and the correlation dimension by changing the strength of the
restoring force. The main result is that there exists a particular value of the
force constant, corresponding to a mean slab depth approximately equal to a few
times the Kolmogorov length scale, that maximizes the clustering of the
particles
A New Robust Regression Method Based on Minimization of Geodesic Distances on a Probabilistic Manifold: Application to Power Laws
In regression analysis for deriving scaling laws that occur in various
scientific disciplines, usually standard regression methods have been applied,
of which ordinary least squares (OLS) is the most popular. In many situations,
the assumptions underlying OLS are not fulfilled, and several other approaches
have been proposed. However, most techniques address only part of the
shortcomings of OLS. We here discuss a new and more general regression method,
which we call geodesic least squares regression (GLS). The method is based on
minimization of the Rao geodesic distance on a probabilistic manifold. For the
case of a power law, we demonstrate the robustness of the method on synthetic
data in the presence of significant uncertainty on both the data and the
regression model. We then show good performance of the method in an application
to a scaling law in magnetic confinement fusion.Comment: Published in Entropy. This is an extended version of our paper at the
34th International Workshop on Bayesian Inference and Maximum Entropy Methods
in Science and Engineering (MaxEnt 2014), 21-26 September 2014, Amboise,
Franc
Confinement-Higgs transition in a disordered gauge theory and the accuracy threshold for quantum memory
We study the +/- J random-plaquette Z_2 gauge model (RPGM) in three spatial
dimensions, a three-dimensional analog of the two-dimensional +/- J random-bond
Ising model (RBIM). The model is a pure Z_2 gauge theory in which randomly
chosen plaquettes (occuring with concentration p) have couplings with the
``wrong sign'' so that magnetic flux is energetically favored on these
plaquettes. Excitations of the model are one-dimensional ``flux tubes'' that
terminate at ``magnetic monopoles.'' Electric confinement can be driven by
thermal fluctuations of the flux tubes, by the quenched background of magnetic
monopoles, or by a combination of the two. Like the RBIM, the RPGM has enhanced
symmetry along a ``Nishimori line'' in the p-T plane (where T is the
temperature). The critical concentration p_c of wrong-sign plaquettes at the
confinement-Higgs phase transition along the Nishimori line can be identified
with the accuracy threshold for robust storage of quantum information using
topological error-correcting codes: if qubit phase errors, qubit bit-flip
errors, and errors in the measurement of local check operators all occur at
rates below p_c, then encoded quantum information can be protected perfectly
from damage in the limit of a large code block. Numerically, we measure p_{c0},
the critical concentration along the T=0 axis (a lower bound on p_c), finding
p_{c0}=.0293 +/- .0002. We also measure the critical concentration of
antiferromagnetic bonds in the two-dimensional RBIM on the T=0 axis, finding
p_{c0}=.1031 +/-.0001. Our value of p_{c0} is incompatible with the value of
p_c=.1093 +/-.0002 found in earlier numerical studies of the RBIM, in
disagreement with the conjecture that the phase boundary of the RBIM is
vertical (parallel to the T axis) below the Nishimori line.Comment: 16 pages, 11 figures, REVTeX, improved numerics and an additional
autho
Properties of a new quasi-axisymmetric configuration
A novel, compact, quasi-axisymmetric configuration is presented which
exhibits low fast-particle losses and is stable to ideal MHD instabilities. The
design has fast-particle loss rates below 8\% for flux surfaces within the
half-radius, and is shown to have an MHD-stability limit of a normalised
pressure of where is volume
averaged. The flux surfaces at various plasma betas and currents as calculated
using the SPEC equilibrium code are presented. Neoclassical transport
coefficients are shown to be similar to an equivalent tokamak, with a distinct
banana regime at half-radius. An initial coil design study is presented to
assess the feasibility of this configuration as a fusion-relevant experiment
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