1,415 research outputs found
Nonequilibrium transport and statistics of Schwinger pair production in Weyl semimetals
The non-equilibrium dynamics beyond linear response of Weyl semimetals is
studied after a sudden switching on of a DC electric field. The resulting
current is a nonmonotonic function of time, with an initial quick increase of
polarization current followed by a power-law decay. Particle-hole creation \`a
la Schwinger dominates for long times when the conduction current takes over
the leading role, with the total current increasing again. The conductivity
estimated from a dynamical calculation within a Drude picture agrees with the
one obtained from Kubo's formula. The full distribution function of
electron-hole pairs changes from Poissonian for short perturbations to a
Gaussian in the long perturbation (Landau-Zener) regime. The vacuum persistence
probability of high energy physics manifests itself in a finite probability of
no pair creation and no induced current at all times.Comment: 7 pages, 4 figure
Sampling-based optimization with mixtures
Sampling-based Evolutionary Algorithms (EA) are of great use when dealing with a highly non-convex and/or noisy optimization task, which is the kind of task we often have to solve in Machine Learning. Two derivative-free examples of such methods are Estimation of Distribution Algorithms (EDA) and techniques based on the Cross-Entropy Method (CEM). One of the main problems these algorithms have to solve is finding a good surrogate model for the normalized target function, that is, a model which has sufficient complexity to fit this target function, but which keeps the computations simple enough. Gaussian mixture models have been applied in practice with great success, but most of these approaches lacked a solid theoretical founding. In this paper we describe a sound mathematical justification for Gaussian mixture surrogate models, more precisely we propose a proper derivation of an EDA/CEM algorithm with mixture updates using Expectation Maximization techniques. It will appear that this algorithm resembles the recent Population MCMC schemes, thus reinforcing the link between Monte- Carlo integration methods and sampling-based optimization. We will concentrate throughout this paper on continuous optimization
Fundamental (f) Oscillations in a Magnetically Coupled Solar Interior-Atmosphere System:An Analytical Approach
Solar fundamental (f) acoustic mode oscillations are investigated analytically in a magnetohydrodynamic (MHD) model. The model consists of three layers in planar geometry, representing the solar interior, the magnetic atmosphere, and a transitional layer sandwiched between them. Since we focus on the fundamental mode here, we assume the plasma is incompressible. A horizontal, canopy-like, magnetic field is introduced to the atmosphere, in which degenerated slow MHD waves can exist. The global (f-mode) oscillations can couple to local atmospheric Alfvén waves, resulting, e.g., in a frequency shift of the oscillations. The dispersion relation of the global oscillation mode is derived, and is solved analytically for the thin-transitional layer approximation and for the weak-field approximation. Analytical formulae are also provided for the frequency shifts due to the presence of a thin transitional layer and a weak atmospheric magnetic field. The analytical results generally indicate that, compared to the fundamental value (ω=gk), the mode frequency is reduced by the presence of an atmosphere by a few per cent. A thin transitional layer reduces the eigen-frequencies further by about an additional hundred microhertz. Finally, a weak atmospheric magnetic field can slightly, by a few percent, increase the frequency of the eigen-mode. Stronger magnetic fields, however, can increase the f-mode frequency by even up to ten per cent, which cannot be seen in observed data. The presence of a magnetic atmosphere in the three-layer model also introduces non-permitted propagation windows in the frequency spectrum; here, f-mode oscillations cannot exist with certain values of the harmonic degree. The eigen-frequencies can be sensitive to the background physical parameters, such as an atmospheric density scale-height or the rate of the plasma density drop at the photosphere. Such information, if ever observed with high-resolution instrumentation and inverted, could help to gain further insight into solar magnetic structures by means of solar magneto-seismology, and could provide further insight into the role of magnetism in solar oscillations
Apportionment and districting by Sum of Ranking Differences
Sum of Ranking Differences is an innovative statistical method that ranks competing solutions based on a reference point. The latter might arise naturally, or can be aggregated from
the data. We provide two case studies to feature both possibilities. Apportionment and districting are two critical issues that emerge in relation to democratic elections. Theoreticians
invented clever heuristics to measure malapportionment and the compactness of the shape
of the constituencies, yet, there is no unique best method in either cases. Using data from
Norway and the US we rank the standard methods both for the apportionment and for the
districting problem. In case of apportionment, we find that all the classical methods perform
reasonably well, with subtle but significant differences. By a small margin the Leximin
method emerges as a winner, but—somewhat unexpectedly—the non-regular Imperiali
method ties for first place. In districting, the Lee-Sallee index and a novel parametric method
the so-called Moment Invariant performs the best, although the latter is sensitive to the function’s chosen parameter
Impact of non-Poisson activity patterns on spreading processes
Halting a computer or biological virus outbreak requires a detailed
understanding of the timing of the interactions between susceptible and
infected individuals. While current spreading models assume that users interact
uniformly in time, following a Poisson process, a series of recent measurements
indicate that the inter-contact time distribution is heavy tailed,
corresponding to a temporally inhomogeneous bursty contact process. Here we
show that the non-Poisson nature of the contact dynamics results in prevalence
decay times significantly larger than predicted by the standard Poisson process
based models. Our predictions are in agreement with the detailed time resolved
prevalence data of computer viruses, which, according to virus bulletins, show
a decay time close to a year, in contrast with the one day decay predicted by
the standard Poisson process based models.Comment: 4 pages, 3 figure
Quantum Monte Carlo Algorithm Based on Two-Body Density Functional Theory for Fermionic Many-Body Systems: Application to 3He
We construct a quantum Monte Carlo algorithm for interacting fermions using
the two-body density as the fundamental quantity. The central idea is mapping
the interacting fermionic system onto an auxiliary system of interacting
bosons. The correction term is approximated using correlated wave functions for
the interacting system, resulting in an effective potential that represents the
nodal surface. We calculate the properties of 3He and find good agreement with
experiment and with other theoretical work. In particular, our results for the
total energy agree well with other calculations where the same approximations
were implemented but the standard quantum Monte Carlo algorithm was usedComment: 4 pages, 3 figures, 1 tabl
Global oscillations in a magnetic solar model:II Oblique propagation
The coupling of solar global acoustic oscillations to a magnetised
solar atmosphere is studied here. The solar interior – atmosphere
interface is modelled by a non-magnetic polytrope interior overlayed
by a planar atmosphere embedded in non-uniform horizontal
atmospheric magnetic field. Pintér & Goossens (1999, A&A, 347, 321) showed that parallel
propagating acoustic waves can couple resonantly to local
magnetohydrodynamic (MHD) slow continuum modes only. In general,
global acoustic modes can, however, propagate in arbitrary
directions with respect to local atmospheric fields giving rise to
an additional efficient coupling mechanism that has consequences on
mode damping and atmospheric energetics. In this paper we study
obliquely propagating global modes that can couple also to local MHD
Alfvén continuum modes. The atmospheric magnetic effects on global
mode frequencies are still much of a debate. In particular, the
resulting frequency shifts and damping rates of global modes caused
by the resonant interaction with both local Alfvén and slow waves
are investigated. We found the coupling of global f and p modes
and the Lamb mode, that penetrate into the magnetic solar
atmosphere, will strongly depend on the direction of propagation
with respect to the solar atmospheric magnetic field. These
frequency shifts, as a function of the propagation direction, give us
a further elegant tool and refinement method of local
helioseismology techniques. Finally we briefly discuss the
importance of studying obliquely propagating waves and discuss the
results in the context of possible helioseismic observations
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