1,373 research outputs found
Electrostatic pair creation and recombination in quantum plasmas
The collective production of electron-positron pairs by electrostatic waves
in quantum plasmas is investigated. In particular, a semi-classical governing
set of equation for a self-consistent treatment of pair creation by the
Schwinger mechanism in a quantum plasma is derived.Comment: 4 pages, 3 figures, to appear in JETP Letter
Biwhitening Reveals the Rank of a Count Matrix
Estimating the rank of a corrupted data matrix is an important task in data
analysis, most notably for choosing the number of components in PCA.
Significant progress on this task was achieved using random matrix theory by
characterizing the spectral properties of large noise matrices. However,
utilizing such tools is not straightforward when the data matrix consists of
count random variables, e.g., Poisson, in which case the noise can be
heteroskedastic with an unknown variance in each entry. In this work, we
consider a Poisson random matrix with independent entries, and propose a simple
procedure termed \textit{biwhitening} for estimating the rank of the underlying
signal matrix (i.e., the Poisson parameter matrix) without any prior knowledge.
Our approach is based on the key observation that one can scale the rows and
columns of the data matrix simultaneously so that the spectrum of the
corresponding noise agrees with the standard Marchenko-Pastur (MP) law,
justifying the use of the MP upper edge as a threshold for rank selection.
Importantly, the required scaling factors can be estimated directly from the
observations by solving a matrix scaling problem via the Sinkhorn-Knopp
algorithm. Aside from the Poisson, our approach is extended to families of
distributions that satisfy a quadratic relation between the mean and the
variance, such as the generalized Poisson, binomial, negative binomial, gamma,
and many others. This quadratic relation can also account for missing entries
in the data. We conduct numerical experiments that corroborate our theoretical
findings, and showcase the advantage of our approach for rank estimation in
challenging regimes. Furthermore, we demonstrate the favorable performance of
our approach on several real datasets of single-cell RNA sequencing
(scRNA-seq), High-Throughput Chromosome Conformation Capture (Hi-C), and
document topic modeling
Interleukin-1: The Pros and Cons of Its Clinical Relevance
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75148/1/j.1525-1594.1988.tb02759.x.pd
Pair creation: back-reactions and damping
We solve the quantum Vlasov equation for fermions and bosons, incorporating
spontaneous pair creation in the presence of back-reactions and collisions.
Pair creation is initiated by an external impulse field and the source term is
non-Markovian. A simultaneous solution of Maxwell's equation in the presence of
feedback yields an internal current and electric field that exhibit plasma
oscillations with a period tau_pl. Allowing for collisions, these oscillations
are damped on a time-scale, tau_r, determined by the collision frequency.
Plasma oscillations cannot affect the early stages of the formation of a
quark-gluon plasma unless tau_r >> tau_pl and tau_pl approx. 1/Lambda_QCD
approx 1 fm/c.Comment: 16 pages, 6 figure, REVTEX, epsfig.st
Dileptons from Disoriented Chiral Condensates
Disoriented chiral condensates or long wavelength pionic oscillations and
their interaction with the thermal environment can be a significant source of
dileptons. We calculate the yield of such dilepton production within the linear
sigma model, both in a quantal mean-field treatment and in a semi-classical
approximation. We then illustrate the basic features of the dilepton spectrum
in a schematic model. We find that dilepton yield with invariant mass near and
below due to the soft pion modes can be up to two orders of
magnitude larger than the corresponding equilibrium yield.Comment: 22 pages, 8 figures, uses epsf-styl
Schwinger Mechanism for Gluon Pair Production in the Presence of Arbitrary Time Dependent Chromo-Electric Field
We study Schwinger mechanism for gluon pair production in the presence of
arbitrary time-dependent chromo-electric background field with
arbitrary color index =1,2,...8 in SU(3) by directly evaluating the path
integral. We obtain an exact expression for the probability of non-perturbative
gluon pair production per unit time per unit volume and per unit transverse
momentum from arbitrary . We show that the
tadpole (or single gluon) effective action does not contribute to the
non-perturbative gluon pair production rate . We find
that the exact result for non-perturbative gluon pair production is independent
of all the time derivatives where
and has the same functional dependence on two casimir invariants
and as the constant
chromo-electric field result with the replacement: . This
result may be relevant to study the production of a non-perturbative
quark-gluon plasma at RHIC and LHC.Comment: 13 pages latex, Published in European Physical Journal
A kinetic approach to eta' production from a CP-odd phase
The production of (eta,eta')- mesons during the decay of a CP-odd phase is
studied within an evolution operator approach. We derive a quantum kinetic
equation starting from the Witten-DiVecchia-Veneziano Lagrangian for
pseudoscalar mesons containing a U_A(1) symmetry breaking term. The non-linear
vacuum mean field for the flavour singlet pseudoscalar meson is treated as a
classical, self-interacting background field with fluctuations assumed to be
small. The numerical solution provides the time evolution of momentum
distribution function of produced eta'- mesons after a quench at the
deconfinement phase transition. We show that the time evolution of the momentum
distribution of the produced mesons depend strongly on the shape of the
effective potential at the end of the quench, exhibiting either parametric or
tachyonic resonances. Quantum statistical effects are essential and lead to a
pronounced Bose enhancement of the low momentum states.Comment: 10 pages, latex, epsfig, 6 figure
Surmising synchrony of sound and sight: Factors explaining variance of audiovisual integration in hurdling, tap dancing and drumming.
Auditory and visual percepts are integrated even when they are not perfectly temporally aligned with each other, especially when the visual signal precedes the auditory signal. This window of temporal integration for asynchronous audiovisual stimuli is relatively well examined in the case of speech, while other natural action-induced sounds have been widely neglected. Here, we studied the detection of audiovisual asynchrony in three different whole-body actions with natural action-induced sounds-hurdling, tap dancing and drumming. In Study 1, we examined whether audiovisual asynchrony detection, assessed by a simultaneity judgment task, differs as a function of sound production intentionality. Based on previous findings, we expected that auditory and visual signals should be integrated over a wider temporal window for actions creating sounds intentionally (tap dancing), compared to actions creating sounds incidentally (hurdling). While percentages of perceived synchrony differed in the expected way, we identified two further factors, namely high event density and low rhythmicity, to induce higher synchrony ratings as well. Therefore, we systematically varied event density and rhythmicity in Study 2, this time using drumming stimuli to exert full control over these variables, and the same simultaneity judgment tasks. Results suggest that high event density leads to a bias to integrate rather than segregate auditory and visual signals, even at relatively large asynchronies. Rhythmicity had a similar, albeit weaker effect, when event density was low. Our findings demonstrate that shorter asynchronies and visual-first asynchronies lead to higher synchrony ratings of whole-body action, pointing to clear parallels with audiovisual integration in speech perception. Overconfidence in the naturally expected, that is, synchrony of sound and sight, was stronger for intentional (vs. incidental) sound production and for movements with high (vs. low) rhythmicity, presumably because both encourage predictive processes. In contrast, high event density appears to increase synchronicity judgments simply because it makes the detection of audiovisual asynchrony more difficult. More studies using real-life audiovisual stimuli with varying event densities and rhythmicities are needed to fully uncover the general mechanisms of audiovisual integration
Chaos in Time Dependent Variational Approximations to Quantum Dynamics
Dynamical chaos has recently been shown to exist in the Gaussian
approximation in quantum mechanics and in the self-consistent mean field
approach to studying the dynamics of quantum fields. In this study, we first
show that any variational approximation to the dynamics of a quantum system
based on the Dirac action principle leads to a classical Hamiltonian dynamics
for the variational parameters. Since this Hamiltonian is generically nonlinear
and nonintegrable, the dynamics thus generated can be chaotic, in distinction
to the exact quantum evolution. We then restrict attention to a system of two
biquadratically coupled quantum oscillators and study two variational schemes,
the leading order large N (four canonical variables) and Hartree (six canonical
variables) approximations. The chaos seen in the approximate dynamics is an
artifact of the approximations: this is demonstrated by the fact that its onset
occurs on the same characteristic time scale as the breakdown of the
approximations when compared to numerical solutions of the time-dependent
Schrodinger equation.Comment: 10 pages (12 figures), RevTeX (plus macro), uses epsf, minor typos
correcte
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