3,191 research outputs found
Bubble formation in potential
Scalar field theory with an asymmetric potential is studied at zero
temperature and high-temperature for potential. The equations of
motion are solved numerically to obtain O(4) spherical symmetric and O(3)
cylindrical symmetric bounce solutions. These solutions control the rates for
tunneling from the false vacuum to the true vacuum by bubble formation. The
range of validity of the thin-wall approximation (TWA) is investigated. An
analytical solution for the bounce is presented, which reproduces the action in
the thin-wall as well as the thick-wall limits.Comment: 22 pag
Glacial-interglacial vegetation dynamics in South Eastern Africa coupled to sea surface temperature variations in the Western Indian Ocean
Glacial-interglacial fluctuations in the vegetation of South Africa might elucidate the climate system at the edge of the tropics between the Indian and Atlantic Oceans. However, vegetation records covering a full glacial cycle have only been published from the eastern South Atlantic. We present a pollen record of the marine core MD96-2048 retrieved by the Marion Dufresne from the Indian Ocean similar to 120 km south of the Limpopo River mouth. The sedimentation at the site is slow and continuous. The upper 6 m (spanning the past 342 Ka) have been analysed for pollen and spores at millennial resolution. The terrestrial pollen assemblages indicate that during interglacials, the vegetation of eastern South Africa and southern Mozambique largely consisted of evergreen and deciduous forests. During glacials open mountainous scrubland dominated. Montane forest with Podocarpus extended during humid periods was favoured by strong local insolation. Correlation with the sea surface temperature record of the same core indicates that the extension of mountainous scrubland primarily depends on sea surface temperatures of the Agulhas Current. Our record corroborates terrestrial evidence of the extension of open mountainous scrubland (including fynbos-like species of the high-altitude Grassland biome) for the last glacial as well as for other glacial periods of the past 300 Ka
Finite-size effects on the chiral phase diagram of four-fermion models in four dimensions
We study the size dependence of the dynamical symmetry breaking in the
four-dimensional Nambu-Jona-Lasinio model. We show that the presence of
boundaries reduces the chiral breaking region, and this effect is strengthened
for a larger number of compactified dimensions. A critical value for the length
of the compactified dimensions exists, below which the dynamical symmetry
breaking is not possible. Considering finite temperature and chemical
potential, the chiral phase structure for the system with compactified
dimensions is obtained. A gradual decreasing of the chiral breaking region with
increasing of chemical potential is found. Also, at fixed chemical potential,
the decreasing of the size of the system changes the order of the chiral phase
transition.Comment: LATEX 14 pages 2 figure
Axion-photon Couplings in Invisible Axion Models
We reexamine the axion-photon couplings in various invisible axion models
motivated by the recent proposal of using optical interferometry at the ASST
facility in the SSCL to search for axion. We illustrate that the assignment of
charges for the fermion fields plays an important role in
determining the couplings. Several simple non-minimal invisible axion models
with suppressed and enhanced axion-photon couplings are constructed,
respectively. We also discuss the implications of possible new experiments to
detect solar axions by conversion to -rays in a static magnetic apparatus
tracking the sun.Comment: 14 pages, LaTeX fil
Incremental learning with social media data to predict near real-time events
International audienceIn this paper, we focus on the problem of predicting some particular user activities in social media. Our challenge is to consider real events such as message posting to friends or forwarding received ones, connecting to new friends, and provide near real-time prediction of new events. Our approach is based on latent factor models which can exploit simultaneously the timestamped interaction information among users and their posted content information. We propose a simple strategy to learn incrementally the latent factors at each time step. Our method takes only recent data to update latent factor models and thus can reduce computational cost. Experiments on a real dataset collected from Twitter show that our method can achieve performances that are comparable with other state-of-the-art non-incremental techniques
Electroactive biofilms: new means for electrochemistry
This work demonstrates that electrochemical reactions can be catalysed by the natural biofilms that form on
electrode surfaces dipping into drinking water or compost. In drinking water, oxygen reduction was monitored with
stainless steel ultra-microelectrodes under constant potential electrolysis at )0.30 V/SCE for 13 days. 16 independent experiments were conducted in drinking water, either pure or with the addition of acetate or dextrose. In
most cases, the current increased and reached 1.5–9.5 times the initial current. The current increase was attributed to
biofilm forming on the electrode in a similar way to that has been observed in seawater. Epifluorescence microscopy
showed that the bacteria size and the biofilm morphology depended on the nutrients added, but no quantitative
correlation between biofilm morphology and current was established. In compost, the oxidation process was
investigated using a titanium based electrode under constant polarisation in the range 0.10–0.70 V/SCE. It was
demonstrated that the indigenous micro-organisms were responsible for the current increase observed after a few
days, up to 60 mA m)2. Adding 10 mM acetate to the compost amplified the current density to 145 mA m)2 at 0.50 V/SCE. The study suggests that many natural environments, other than marine sediments, waste waters and
seawaters that have been predominantly investigated until now, may be able to produce electrochemically active
biofilm
Fluence Dependence of Charge Collection of irradiated Pixel Sensors
The barrel region of the CMS pixel detector will be equipped with ``n-in-n''
type silicon sensors. They are processed on DOFZ material, use the moderated
p-spray technique and feature a bias grid. The latter leads to a small fraction
of the pixel area to be less sensitive to particles. In order to quantify this
inefficiency prototype pixel sensors irradiated to particle fluences between
and 2.6\times 10^{15} \Neq have been bump bonded to
un-irradiated readout chips and tested using high energy pions at the H2 beam
line of the CERN SPS. The readout chip allows a non zero suppressed analogue
readout and is therefore well suited to measure the charge collection
properties of the sensors.
In this paper we discuss the fluence dependence of the collected signal and
the particle detection efficiency. Further the position dependence of the
efficiency is investigated.Comment: 11 Pages, Presented at the 5th Int. Conf. on Radiation Effects on
Semiconductor Materials Detectors and Devices, October 10-13, 2004 in
Florence, Italy, v3: more typos corrected, minor changes required by the
refere
Dissolution in a field
We study the dissolution of a solid by continuous injection of reactive
``acid'' particles at a single point, with the reactive particles undergoing
biased diffusion in the dissolved region. When acid encounters the substrate
material, both an acid particle and a unit of the material disappear. We find
that the lengths of the dissolved cavity parallel and perpendicular to the bias
grow as t^{2/(d+1)} and t^{1/(d+1)}, respectively, in d-dimensions, while the
number of reactive particles within the cavity grows as t^{2/(d+1)}. We also
obtain the exact density profile of the reactive particles and the relation
between this profile and the motion of the dissolution boundary. The extension
to variable acid strength is also discussed.Comment: 6 pages, 6 figures, 2-column format, for submission to PR
Permeability and conductivity of platelet-reinforced membranes and composites
We present large scale simulations of the diffusion constant of a random
composite consisting of aligned platelets with aspect ratio in a
matrix (with diffusion constant ) and find that , where and is the platelet volume fraction. We
demonstrate that for large aspect ratio platelets the pair term ()
dominates suggesting large property enhancements for these materials. However a
small amount of face-to-face ordering of the platelets markedly degrades the
efficiency of platelet reinforcement.Comment: RevTeX, 5 pages, 4 figures, submitted to PR
Generalized Phase Synchronization in unidirectionally coupled chaotic oscillators
We investigate phase synchronization between two identical or detuned
response oscillators coupled to a slightly different drive oscillator. Our
result is that phase synchronization can occur between response oscillators
when they are driven by correlated (but not identical) inputs from the drive
oscillator. We call this phenomenon Generalized Phase Synchronization (GPS) and
clarify its characteristics using Lyapunov exponents and phase difference
plots.Comment: 4 pages, 5 figure
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