1,739 research outputs found
Synchronized oscillations and acoustic fluidization in confined granular materials
According to the acoustic fluidization hypothesis, elastic waves at a
characteristic frequency form inside seismic faults even in the absence of an
external perturbation. These waves are able to generate a normal stress which
contrasts the confining pressure and promotes failure. Here, we study the
mechanisms responsible for this wave activation via numerical simulations of a
granular fault model. We observe the particles belonging to the percolating
backbone, which sustains the stress, to perform synchronized oscillations over
ellipticlike trajectories in the fault plane. These oscillations occur at the
characteristic frequency of acoustic fluidization. As the applied shear stress
increases, these oscillations become perpendicular to the fault plane just
before the system fails, opposing the confining pressure, consistently with the
acoustic fluidization scenario. The same change of orientation can be induced
by external perturbations at the acoustic fluidization frequency
Induced and endogenous acoustic oscillations in granular faults
The frictional properties of disordered systems are affected by external
perturbations. These perturbations usually weaken the system by reducing the
macroscopic friction coefficient. This friction reduction is of particular
interest in the case of disordered systems composed of granular particles
confined between two plates, as this is a simple model of seismic fault.
Indeed, in the geophysical context frictional weakening could explain the
unexpected weakness of some faults, as well as earthquake remote triggering. In
this manuscript we review recent results concerning the response of confined
granular systems to external perturbations, considering the different
mechanisms by which the perturbation could weaken a system, the relevance of
the frictional reduction to earthquakes, as well as discussing the intriguing
scenario whereby the weakening is not monotonic in the perturbation frequency,
so that a re-entrant transition is observed, as the system first enters a
fluidized state and then returns to a frictional state.Comment: 15 pages, 12 figure
Conformal vs confining scenario in SU(2) with adjoint fermions
The masses of the lowest-lying states in the meson and in the gluonic sector
of an SU(2) gauge theory with two Dirac flavors in the adjoint representation
are measured on the lattice at a fixed value of the lattice coupling for values of the bare fermion mass that span a range
between the quenched regime and the massless limit, and for various lattice
volumes. Even for light constituent fermions the lightest glueballs are found
to be lighter than the lightest mesons. Moreover, the string tension between
two static fundamental sources strongly depends on the mass of the dynamical
fermions and becomes of the order of the inverse squared lattice linear size
before the chiral limit is reached. The implications of these findings for the
phase of the theory in the massless limit are discussed and a strategy for
discriminating between the (near--)conformal and the confining scenario is
outlined.Comment: 5 pages, 4 figures using RevTeX4, Typos corrected, references added.
Versions to appear on PR
Dynamical Correlation Length and Relaxation Processes in a Glass Former
We investigate the relaxation process and the dynamical heterogeneities of
the kinetically constrained Kob--Anderson lattice glass model, and show that
these are characterized by different timescales. The dynamics is well described
within the diffusing defect paradigm, which suggest to relate the relaxation
process to a reverse--percolation transition. This allows for a geometrical
interpretation of the relaxation process, and of the different timescales
Topological susceptibility in the SU(3) gauge theory
We compute the topological susceptibility for the SU(3) Yang--Mills theory by
employing the expression of the topological charge density operator suggested
by Neuberger's fermions. In the continuum limit we find r_0^4 chi = 0.059(3),
which corresponds to chi=(191 +/- 5 MeV)^4 if F_K is used to set the scale. Our
result supports the Witten--Veneziano explanation for the large mass of the
eta'.Comment: Final version to appear on Phys. Rev. Let
Praziquantel: its use in control of schistosomiasis in sub-Saharan Africa and current research needs
Treatment with praziquantel (PZQ) has become virtually the sole basis of schistosomiasis control in sub-Saharan Africa and elsewhere, and the drug is reviewed here in the context of the increasing rate that it is being used for this purpose. Attention is drawn to our relative lack of knowledge about the mechanisms of action of PZQ at the molecular level, the need for more work to be done on schistosome isolates that have been collected recently from endemic areas rather than those maintained in laboratory conditions for long periods, and our reliance for experimental work mainly on Schistosoma mansoni, little work having been done on S. haematobium. There is no evidence that resistance to PZQ has been induced in African schistosomes as a result of its large-scale use on that continent to date, but there is also no assurance that PZQ and/or schistosomes are in any way unique and that resistant organisms will not be selected as a result of widespread drug usage. The failure of PZQ to produce complete cures in populations given a routine treatment should therefore solicit considerable concern. With few alternatives to PZQ currently available and/or on the horizon, methods to monitor drug-susceptibility in African schistosomes need to be devised and used to help ensure that this drug remains effective for as long a time as possibl
Dynamics of drag and force distributions for projectile impact in a granular medium
Our experiments and molecular dynamics simulations on a projectile
penetrating a two-dimensional granular medium reveal that the mean deceleration
of the projectile is constant and proportional to the impact velocity. Thus,
the time taken for a projectile to decelerate to a stop is independent of its
impact velocity. The simulations show that the probability distribution
function of forces on grains is time-independent during a projectile's
penetration of the medium. At all times the force distribution function
decreases exponentially for large forces.Comment: 4 page
Correlations and Omori law in Spamming
The most costly and annoying characteristic of the e-mail communication
system is the large number of unsolicited commercial e-mails, known as spams,
that are continuously received. Via the investigation of the statistical
properties of the spam delivering intertimes, we show that spams delivered to a
given recipient are time correlated: if the intertime between two consecutive
spams is small (large), then the next spam will most probably arrive after a
small (large) intertime. Spam temporal correlations are reproduced by a
numerical model based on the random superposition of spam sequences, each one
described by the Omori law. This and other experimental findings suggest that
statistical approaches may be used to infer how spammers operate.Comment: Europhysics Letters, to appea
Topological susceptibility from the overlap
The chiral symmetry at finite lattice spacing of Ginsparg-Wilson fermionic
actions constrains the renormalization of the lattice operators; in particular,
the topological susceptibility does not require any renormalization, when using
a fermionic estimator to define the topological charge. Therefore, the overlap
formalism appears as an appealing candidate to study the continuum limit of the
topological susceptibility while keeping the systematic errors under
theoretical control. We present results for the SU(3) pure gauge theory using
the index of the overlap Dirac operator to study the topology of the gauge
configurations. The topological charge is obtained from the zero modes of the
overlap and using a new algorithm for the spectral flow analysis. A detailed
comparison with cooling techniques is presented. Particular care is taken in
assessing the systematic errors. Relatively high statistics (500 to 1000
independent configurations) yield an extrapolated continuum limit with errors
that are comparable with other methods. Our current value from the overlap is
\chi^{1/4} = 188 \pm 12 \pm 5 \MeV.Comment: 18 pages, 7 figure
Linking smallholders to livestock markets: Combining market and household survey data in Tanzania
Linking farmers to markets is widely viewed as a milestone towards promoting economic growth
and poverty reduction. However, market and institutional imperfections along the supply chain
thwart perfect vertical and spatial price transmission and prevent farmers and market actors from
getting access to information, identifying business opportunities and allocating their resources
efficiently. This acts as a barrier to market-led rural development and poverty reduction. This paper
reviews and analyses household information, and the major livestock market and marketing data
available in Tanzania, in relation to market-led development possibilities. Household-level data
collected by the Tanzania National Bureau of Statistics and market data collected and disseminated
by the Livestock Information and Knowledge System of the Tanzania Ministry of Industry and
Trade are reviewed and utilised together. Both types of data help identify market opportunities for
livestock producers, but only their joint use could provide policy makers with the information
needed to design and implement policies that facilitate access to markets for livestock producers.
Options to promote integration of household-level data and market data are discussed, which would
facilitate the implementation of the Tanzania Statistical Master Plan and contribute to the
implementation of the Global Strategy to Improve Agricultural and Rural Statistic
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