1,284 research outputs found
Iraq today: The failure of re-shaping a state on sectarian and quota lines.
By Professor Saad N Jawad Senior Research Fellow at the Middle East Centre, LSE & Dr Sawsan I al-Assaf Board Member of the Peace Building-Academy for the Middle East, Spain & Beirut
A note on Gauge Theories Coupled to Gravity
We analyze the bound on gauge couplings , suggested by
Arkani-Hamed et.al. We show this bound can be derived from simple
semi-classical considerations and holds in spacetime dimensions greater than or
equal to four. Non abelian gauge symmetries seem to satisfy the bound in a
trivial manner. We comment on the case of discrete symmetries and close by
performing some checks for the bound in higher dimensions in the context of
string theory.Comment: 15 pages, 1 figure, Late
Extinction of metastable stochastic populations
We investigate extinction of a long-lived self-regulating stochastic
population, caused by intrinsic (demographic) noise. Extinction typically
occurs via one of two scenarios depending on whether the absorbing state n=0 is
a repelling (scenario A) or attracting (scenario B) point of the deterministic
rate equation. In scenario A the metastable stochastic population resides in
the vicinity of an attracting fixed point next to the repelling point n=0. In
scenario B there is an intermediate repelling point n=n_1 between the
attracting point n=0 and another attracting point n=n_2 in the vicinity of
which the metastable population resides. The crux of the theory is WKB method
which assumes that the typical population size in the metastable state is
large. Starting from the master equation, we calculate the quasi-stationary
probability distribution of the population sizes and the (exponentially long)
mean time to extinction for each of the two scenarios. When necessary, the WKB
approximation is complemented (i) by a recursive solution of the
quasi-stationary master equation at small n and (ii) by the van Kampen
system-size expansion, valid near the fixed points of the deterministic rate
equation. The theory yields both entropic barriers to extinction and
pre-exponential factors, and holds for a general set of multi-step processes
when detailed balance is broken. The results simplify considerably for
single-step processes and near the characteristic bifurcations of scenarios A
and B.Comment: 19 pages, 7 figure
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Health systems and policy research evidence in health policy making in Israel: what are researchers’ practices in transferring knowledge to policy makers?
Background: Ensuring the use of research evidence in health system management and policy decisions is an important challenge in this century. Knowledge transfer and exchange (KTE) has emerged as a paradigm to address the challenges and start closing the ‘know-do’ gap. This area of work is gaining momentum in most developed countries, yet, to date, no work has been performed in Israel within this area. The purpose of this study was to identify which KTE activities health systems and policy researchers in Israel have undertaken. Methods: A cross-sectional web-based survey of researchers who have conducted health systems and policy research in Israel was developed. The survey consisted of a demographics section, quantitative scales, and open-ended questions. The survey was sent to all health systems and policy researchers in Israel (n = 125). Results: The study response rate (28%) was relatively low as compared to other studies in the same field (range of 42% to 88%). Our survey found that more than a third of the health systems and policy researchers in Israel reported that they were frequently or always involved in the following KTE activities: interactions with target audience through the research process (i.e., during developing a research question or executing the research; 35% to 42%) or through formal or informal meetings during conferences, workshops, or conversations (40%). Less than half of the health systems and policy researchers in Israel are engaged in bridging activities aimed to facilitate target audiences to use research. Conclusions: This is a fairly new area in Israel and therefore the level of engagement of researchers in KTE activities is not very high. The low response rates could be because KTE is a new field in Israel and minimal KTE initiatives have been undertaken. It is preferable to have higher response rates, yet, after several initiatives, this was the outcome. While the findings are relevant, they may not reflect the total population of health system and policy researchers in Israel. Health system and policy researchers in Israel need to be introduced to the benefits and potential advantages of KTE in an organized and systematic way
On population extinction risk in the aftermath of a catastrophic event
We investigate how a catastrophic event (modeled as a temporary fall of the
reproduction rate) increases the extinction probability of an isolated
self-regulated stochastic population. Using a variant of the Verhulst logistic
model as an example, we combine the probability generating function technique
with an eikonal approximation to evaluate the exponentially large increase in
the extinction probability caused by the catastrophe. This quantity is given by
the eikonal action computed over "the optimal path" (instanton) of an effective
classical Hamiltonian system with a time-dependent Hamiltonian. For a general
catastrophe the eikonal equations can be solved numerically. For simple models
of catastrophic events analytic solutions can be obtained. One such solution
becomes quite simple close to the bifurcation point of the Verhulst model. The
eikonal results for the increase in the extinction probability caused by a
catastrophe agree well with numerical solutions of the master equation.Comment: 11 pages, 11 figure
Experimental Study of Parametric Autoresonance in Faraday Waves
The excitation of large amplitude nonlinear waves is achieved via parametric
autoresonance of Faraday waves. We experimentally demonstrate that phase
locking to low amplitude driving can generate persistent high-amplitude growth
of nonlinear waves in a dissipative system. The experiments presented are in
excellent agreement with theory.Comment: 4 pages, 4 eps figures, to appear in Phys. Rev. Let
First Passage Distributions in a Collective Model of Anomalous Diffusion with Tunable Exponent
We consider a model system in which anomalous diffusion is generated by
superposition of underlying linear modes with a broad range of relaxation
times. In the language of Gaussian polymers, our model corresponds to Rouse
(Fourier) modes whose friction coefficients scale as wavenumber to the power
. A single (tagged) monomer then executes subdiffusion over a broad range
of time scales, and its mean square displacement increases as with
. To demonstrate non-trivial aspects of the model, we numerically
study the absorption of the tagged particle in one dimension near an absorbing
boundary or in the interval between two such boundaries. We obtain absorption
probability densities as a function of time, as well as the position-dependent
distribution for unabsorbed particles, at several values of . Each of
these properties has features characterized by exponents that depend on
. Characteristic distributions found for different values of
have similar qualitative features, but are not simply related quantitatively.
Comparison of the motion of translocation coordinate of a polymer moving
through a pore in a membrane with the diffusing tagged monomer with identical
also reveals quantitative differences.Comment: LaTeX, 10 pages, 8 eps figure
Attempted density blowup in a freely cooling dilute granular gas: hydrodynamics versus molecular dynamics
It has been recently shown (Fouxon et al. 2007) that, in the framework of
ideal granular hydrodynamics (IGHD), an initially smooth hydrodynamic flow of a
granular gas can produce an infinite gas density in a finite time. Exact
solutions that exhibit this property have been derived. Close to the
singularity, the granular gas pressure is finite and almost constant. This work
reports molecular dynamics (MD) simulations of a freely cooling gas of nearly
elastically colliding hard disks, aimed at identifying the "attempted" density
blowup regime. The initial conditions of the simulated flow mimic those of one
particular solution of the IGHD equations that exhibits the density blowup. We
measure the hydrodynamic fields in the MD simulations and compare them with
predictions from the ideal theory. We find a remarkable quantitative agreement
between the two over an extended time interval, proving the existence of the
attempted blowup regime. As the attempted singularity is approached, the
hydrodynamic fields, as observed in the MD simulations, deviate from the
predictions of the ideal solution. To investigate the mechanism of breakdown of
the ideal theory near the singularity, we extend the hydrodynamic theory by
accounting separately for the gradient-dependent transport and for finite
density corrections.Comment: 11 pages, 9 figures, accepted for publication on Physical Review
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