11,214 research outputs found
Supersymmetric Langevin equation to explore free energy landscapes
The recently discovered supersymmetric generalizations of Langevin dynamics
and Kramers equation can be utilized for the exploration of free energy
landscapes of systems whose large time-scale separation hampers the usefulness
of standard molecular dynamics techniques. The first realistic application is
here presented. The system chosen is a minimalist model for a short alanine
peptide exhibiting a helix-coil transition.Comment: 9 pages, 9 figures, RevTeX 4 v2: conclusive section enlarged,
references adde
Role of mycobacteria-induced monocyte/macrophage apoptosis in the pathogenesis of human tuberculosis
The Wishart short rate model
We consider a short rate model, driven by a stochastic process on the cone of
positive semidefinite matrices. We derive sufficient conditions ensuring that
the model replicates normal, inverse or humped yield curves
Cortical free association dynamics: distinct phases of a latching network
A Potts associative memory network has been proposed as a simplified model of
macroscopic cortical dynamics, in which each Potts unit stands for a patch of
cortex, which can be activated in one of S local attractor states. The internal
neuronal dynamics of the patch is not described by the model, rather it is
subsumed into an effective description in terms of graded Potts units, with
adaptation effects both specific to each attractor state and generic to the
patch. If each unit, or patch, receives effective (tensor) connections from C
other units, the network has been shown to be able to store a large number p of
global patterns, or network attractors, each with a fraction a of the units
active, where the critical load p_c scales roughly like p_c ~ (C S^2)/(a
ln(1/a)) (if the patterns are randomly correlated). Interestingly, after
retrieving an externally cued attractor, the network can continue jumping, or
latching, from attractor to attractor, driven by adaptation effects. The
occurrence and duration of latching dynamics is found through simulations to
depend critically on the strength of local attractor states, expressed in the
Potts model by a parameter w. Here we describe with simulations and then
analytically the boundaries between distinct phases of no latching, of
transient and sustained latching, deriving a phase diagram in the plane w-T,
where T parametrizes thermal noise effects. Implications for real cortical
dynamics are briefly reviewed in the conclusions
Improved limits on photon velocity oscillations
The mixing of the photon with a hypothetical sterile paraphotonic state would
have consequences on the cosmological propagation of photons. The absence of
distortions in the optical spectrum of distant Type Ia supernov\ae allows to
extend by two orders of magnitude the previous limit on the Lorentz-violating
parameter associated to the photon-paraphoton transition, extracted
from the abscence of distortions in the spectrum of the cosmic microwave
background. The new limit is consistent with the interpretation of the dimming
of distant Type Ia supernov\ae as a consequence of a nonzero cosmological
constant. Observations of gamma-rays from active galactic nuclei allow to
further extend the limit on by ten orders of magnitude.Comment: 10 pages, 4 Postscript figures, use epsfig, amssym
External Capital Structures and Oil Price Volatility
This paper assesses the extent to which a country’s external capital structure can aid in mitigating the macroeconomic impact of oil price shocks. Two Caribbean economies highly vulnerable to oil price shocks are considered: an oil importer (Jamaica) and an oil exporter (Trinidad and Tobago). From a risk-sharing perspective, a desirable external capital structure is one that, through international capital gains and losses, helps offset responses of the current account balance to external shocks. It is found that both countries could alter their international portfolio to provide a better buffer against such shocks.Hedging, Oil, Foreign assets and liabilities, International portfolios
L\'evy walks and scaling in quenched disordered media
We study L\'evy walks in quenched disordered one-dimensional media, with
scatterers spaced according to a long-tailed distribution. By analyzing the
scaling relations for the random-walk probability and for the resistivity in
the equivalent electric problem, we obtain the asymptotic behavior of the mean
square displacement as a function of the exponent characterizing the scatterers
distribution. We demonstrate that in quenched media different average
procedures can display different asymptotic behavior. In particular, we
estimate the moments of the displacement averaged over processes starting from
scattering sites, in analogy with recent experiments. Our results are compared
with numerical simulations, with excellent agreement.Comment: Phys. Rev. E 81, 060101(R) (2010
Cerebral venous hemodynamic abnormalities in episodic and chronic migraine
Alterations of cerebral venous drainage have been demonstrated in chronic migraine (CM), suggesting that cerebral venous hemodynamic abnormalities (CVHAs) play a role in this condition. The aim of the present study was to look for a correlation between CM and CVHAs. We recruited 33 subjects suffering from CM with or without analgesic overuse, 29 episodic migraine (EM) patients with or without aura, and 21 healthy subjects as controls (HCs). CVHAs were evaluated by transcranial and extracranial echo-color Doppler evaluation of five venous hemodynamic parameters. CVHAs were significantly more frequent in the CM and EM patients than in the HCs. In the migraine patients, CVHAs were not correlated with clinical features. Cerebral venous hemodynamic abnormalities in episodic and chronic migraine The significantly greater frequency of CVHAs observed in the migraineurs may reflect a possible relationship between migraine and these abnormalities. Prospective longitudinal studies are needed to investigate whether CVHAs have a role in the processes of migraine chronification
Multiphoton Processes in Driven Mesoscopic Systems
We study the statistics of multi-photon absorption/emission processes in a
mesoscopic ring threaded by an harmonic time-dependent flux . For this
sake, we demonstrate a useful analogy between the Keldysh quantum kinetic
equation for the electrons distribution function and a Continuous Time Random
Walk in energy space with corrections due to interference effects. Studying the
probability to absorb/emit quanta per scattering event, we
explore the crossover between ultra-quantum/low-intensity limit and
quasi-classical/high-intensity regime, and the role of multiphoton processes in
driving it.Comment: 6 pages, 5 figures, extended versio
Breaking the AMSP mould: the increasingly strange case of HETE J1900.1-2455
We present ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations
of the 377.3 Hz accretion-powered pulsar, HETE J1900.1-2455 Activity continues
in this system more than 3 years after discovery, at a mean luminosity of
4.4e36 erg/s (for d=5 kpc), although pulsations were present only within the
first 70 days. X-ray variability has increased each year, notably with a brief
interval of nondetection in 2007, during which the luminosity dropped to below
1e-3 of the mean level. A deep search of data from the intervals of
nondetection in 2005 revealed evidence for extremely weak pulsations at an
amplitude of 0.29% rms, a factor of ten less than the largest amplitude seen
early in the outburst.
X-ray burst activity continued through 2008, with bursts typically featuring
strong radius expansion. Spectral analysis of the most intense burst detected
by RXTE early in the outburst revealed unusual variations in the inferred
photospheric radius, as well as significant deviations from a blackbody. We
obtained much better fits instead with a comptonisation model.Comment: 8 pages, 3 figures, one table; to appear in the proceedings of the
workshop "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April
2008, eds. R. Wijnands et al. (AIP Conf. Proc.
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