18,676 research outputs found
Impact of Dynamical Fermions on QCD Vacuum Structure
We examine how dynamical fermions affect both the UV and infrared structure
of the QCD vacuum. We consider large lattices from the MILC
collaboration, using a gluonic definition of the topological charge density,
founded on a new over-improved stout-link smearing algorithm. The algorithm
reproduces established results from the overlap formalism and is designed to
preserve nontrivial topological objects including instantons. At short
distances we focus on the topological charge correlator, , where
negative values at small $x$ reveal a sign-alternating layered structure to the
topological-charge density of the QCD vacuum. We find that the magnitudes of
the negative dip in the correlator and the positive
contact term are both increased with the introduction of dynamical fermion
degrees of freedom. This is in accord with expectations based on charge
renormalization and the vanishing of the topological susceptibility in the
chiral limit. At large distances we examine the extent to which instanton-like
objects are found on the lattice, and how their distributions vary between
quenched and dynamical gauge fields. We show that dynamical gauge fields
contain more instanton-like objects with an average size greater than in the
quenched vacuum. Finally, we directly visualize the topological charge density
in order to investigate the effects of dynamical sea-quark degrees of freedom
on topology.Comment: 9 pages, 8 figure
X-ray Emission from the Weak-lined T Tauri Binary System KH 15D
The unique eclipsing, weak-lined T Tauri star KH 15D has been detected as an
X-ray source in a 95.7 ks exposure from the Chandra X-ray Observatory archives.
A maximum X-ray luminosity of 1.5 x 10^{29} erg s is derived in the
0.5--8 keV band, corresponding to L_{X}/L_bol = 7.5 x 10^{-5}. Comparison with
samples of stars of similar effective temperature in NGC 2264 and in the Orion
Nebula Cluster shows that this is about an order of magnitude low for a typical
star of its mass and age. We argue that the relatively low luminosity cannot be
attributed to absorption along the line of sight but implies a real deficiency
in X-ray production. Possible causes for this are considered in the context of
a recently proposed eccentric binary model for KH 15D. In particular, we note
that the visible component rotates rather slowly for a weak-lined T Tauri star
and has possibly been pseudosynchronized by tidal interaction with the primary
near periastron
Frequency-dependent fitness induces multistability in coevolutionary dynamics
Evolution is simultaneously driven by a number of processes such as mutation,
competition and random sampling. Understanding which of these processes is
dominating the collective evolutionary dynamics in dependence on system
properties is a fundamental aim of theoretical research. Recent works
quantitatively studied coevolutionary dynamics of competing species with a
focus on linearly frequency-dependent interactions, derived from a
game-theoretic viewpoint. However, several aspects of evolutionary dynamics,
e.g. limited resources, may induce effectively nonlinear frequency
dependencies. Here we study the impact of nonlinear frequency dependence on
evolutionary dynamics in a model class that covers linear frequency dependence
as a special case. We focus on the simplest non-trivial setting of two
genotypes and analyze the co-action of nonlinear frequency dependence with
asymmetric mutation rates. We find that their co-action may induce novel
metastable states as well as stochastic switching dynamics between them. Our
results reveal how the different mechanisms of mutation, selection and genetic
drift contribute to the dynamics and the emergence of metastable states,
suggesting that multistability is a generic feature in systems with
frequency-dependent fitness.Comment: 12 pages, 6 figures; J. R. Soc. Interface (2012
Procalcitonin as a biomarker for infection and sepsis: yet again
John L Moran, and Patricia J Solomo
Volatility in high-frequency intensive care mortality time series: application of univariate and multivariate GARCH models
Mortality time series display time-varying volatility. The utility of statistical estimators from the financial time-series paradigm, which account for this characteristic, has not been addressed for high-frequency mortality series. Using daily mean-mortality series of an exemplar intensive care unit (ICU) from the Australian and New Zealand Intensive Care Society adult patient database, joint estimation of a mean and conditional variance (volatility) model for a stationary series was undertaken via univariate autoregressive moving average (ARMA, lags (p, q)), GARCH (Generalised Autoregressive Conditional Heteroscedasticity, lags (p, q)). The temporal dynamics of the conditional variance and correlations of multiple provider series, from rural/ regional, metropolitan, tertiary and private ICUs, were estimated utilising multivariate GARCH models. For the stationary first differenced series, an asymmetric power GARCH model (lags (1, 1)) with t distribution (degrees-offreedom, 11.6) and ARMA (7,0) for the mean-model, was the best-fitting. The four multivariate component series demonstrated varying trend mortality decline and persistent autocorrelation. Within each MGARCH series no model specification dominated. The conditional correlations were surprisingly low (<0.1) between tertiary series and substantial (0.4 - 0.6) between rural-regional and private series. The conditional-variances of both the univariate and multivariate series demonstrated a slow rate of time decline from periods of early volatility and volatility spikes.John L. Moran, Patricia J. Solomo
The mass ratio distribution of short period double degenerate stars
Short period double degenerates (DDs) are close white dwarf - white dwarf
binary stars which are the result of the evolution of interacting binary stars.
We present the first definitive measurements of the mass ratio for two DDs,
WD0136+768 and WD1204+450, and an improved measurement of the mass ratio for
WD0957-666. We compare the properties of the 6 known DDs with measured mass
ratios to the predictions of various theoretical models. We confirm the result
that standard models for the formation of DDs do not predict sufficient DDs
with mass ratios near 1. We also show that the observed difference in cooling
ages between white dwarfs in DDs is a useful constraint on the initial mass
ratio of the binary. A more careful analysis of the properties of the white
dwarf pair WD1704+481.2 leads us to conclude that the brighter white dwarf is
older than its fainter companion. This is the opposite of the usual case for
DDs and is caused by the more massive white dwarf being smaller and cooling
faster. The mass ratio in the sense (mass of younger star)/(mass of older star)
is then 1.43+-0.06 rather than the value 0.70+-0.03 given previously.Comment: Accepted for publication in MNRA
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