13,316 research outputs found
Electromagnetic interactions of muons
The electromagnetic interactions of muons in iron and lead have been studied, using cosmic ray muons, at Durham, 198 feet above sea level, and at London, 60 m.w.e. underground, respectively. The energies of the secondary particles from the interactions were determined from the cascade showers which these particles produced in layers of absorber. As a result of these studies it has been shown that, over the range of transferred energies from 0.1 GeV to 30 GeV, there is no discrepancy between- the results predicted on the basis of quantum electrodynamics and the experimental results. Since the production of knock-on electrons is the dominant process over most of this transferred energy range, there is thus no evidence for a breakdown of quantum electrodynamical theory in close collisions between muons and electrons. By combining this result with the result of an accurate measurement of the anomalous magnetic moment of the muon it is further concluded that, at the present time, there is no evidence for the muon differing from a heavy electron
Study of porous wall low density wind tunnel diffusers
Porous wall wind tunnel diffusers used with low density hypersonic nozzl
A generalization of Hausdorff dimension applied to Hilbert cubes and Wasserstein spaces
A Wasserstein spaces is a metric space of sufficiently concentrated
probability measures over a general metric space. The main goal of this paper
is to estimate the largeness of Wasserstein spaces, in a sense to be precised.
In a first part, we generalize the Hausdorff dimension by defining a family of
bi-Lipschitz invariants, called critical parameters, that measure largeness for
infinite-dimensional metric spaces. Basic properties of these invariants are
given, and they are estimated for a naturel set of spaces generalizing the
usual Hilbert cube. In a second part, we estimate the value of these new
invariants in the case of some Wasserstein spaces, as well as the dynamical
complexity of push-forward maps. The lower bounds rely on several embedding
results; for example we provide bi-Lipschitz embeddings of all powers of any
space inside its Wasserstein space, with uniform bound and we prove that the
Wasserstein space of a d-manifold has "power-exponential" critical parameter
equal to d.Comment: v2 Largely expanded version, as reflected by the change of title; all
part I on generalized Hausdorff dimension is new, as well as the embedding of
Hilbert cubes into Wasserstein spaces. v3 modified according to the referee
final remarks ; to appear in Journal of Topology and Analysi
Bubbling the False Vacuum Away
We investigate the role of nonperturbative, bubble-like inhomogeneities on
the decay rate of false-vacuum states in two and three-dimensional scalar field
theories. The inhomogeneities are induced by setting up large-amplitude
oscillations of the field about the false vacuum as, for example, after a rapid
quench or in certain models of cosmological inflation. We show that, for a wide
range of parameters, the presence of large-amplitude bubble-like
inhomogeneities greatly accelerates the decay rate, changing it from the
well-known exponential suppression of homogeneous nucleation to a power-law
suppression. It is argued that this fast, power-law vacuum decay -- known as
resonant nucleation -- is promoted by the presence of long-lived oscillons
among the nonperturbative fluctuations about the false vacuum. A phase diagram
is obtained distinguishing three possible mechanisms for vacuum decay:
homogeneous nucleation, resonant nucleation, and cross-over. Possible
applications are briefly discussed.Comment: 13 Pages, 16 figures, revtex4, submitted to pr
Global data for ecology and epidemiology: a novel algorithm for temporal Fourier processing MODIS data
Background. Remotely-sensed environmental data from earth-orbiting satellites are increasingly used to model the distribution and abundance of both plant and animal species, especially those of economic or conservation importance. Time series of data from the MODerate-resolution Imaging Spectroradiometer (MODIS) sensors on-board NASA's Terra and Aqua satellites offer the potential to capture environmental thermal and vegetation seasonality, through temporal Fourier analysis, more accurately than was previously possible using the NOAA Advanced Very High Resolution Radiometer (AVHRR) sensor data. MODIS data are composited over 8- or 16-day time intervals that pose unique problems for temporal Fourier analysis. Applying standard techniques to MODIS data can introduce errors of up to 30% in the estimation of the amplitudes and phases of the Fourier harmonics. Methodology/Principal Findings. We present a novel spline-based algorithm that overcomes the processing problems of composited MODIS data. The algorithm is tested on artificial data generated using randomly selected values of both amplitudes and phases, and provides an accurate estimate of the input variables under all conditions. The algorithm was then applied to produce layers that capture the seasonality in MODIS data for the period from 2001 to 2005. Conclusions/Significance. Global temporal Fourier processed images of 1 km MODIS data for Middle Infrared Reflectance, day- and night-time Land Surface Temperature (LST), Normalised Difference Vegetation Index (NDVI), and Enhanced Vegetation Index (EVI) are presented for ecological and epidemiological applications. The finer spatial and temporal resolution, combined with the greater geolocational and spectral accuracy of the MODIS instruments, compared with previous multi-temporal data sets, mean that these data may be used with greater confidence in species' distribution modelling
Autocatalytic plume pinch-off
A localized source of buoyancy flux in a non-reactive fluid medium creates a
plume. The flux can be provided by either heat, a compositional difference
between the fluid comprising the plume and its surroundings, or a combination
of both. For autocatalytic plumes produced by the iodate-arsenous acid
reaction, however, buoyancy is produced along the entire reacting interface
between the plume and its surroundings. Buoyancy production at the moving
interface drives fluid motion, which in turn generates flow that advects the
reaction front. As a consequence of this interplay between fluid flow and
chemical reaction, autocatalytic plumes exhibit a rich dynamics during their
ascent through the reactant medium. One of the more interesting dynamical
features is the production of an accelerating vortical plume head that in
certain cases pinches-off and detaches from the upwelling conduit. After
pinch-off, a new plume head forms in the conduit below, and this can lead to
multiple generations of plume heads for a single plume initiation. We
investigated the pinch-off process using both experimentation and simulation.
Experiments were performed using various concentrations of glycerol, in which
it was found that repeated pinch-off occurs exclusively in a specific
concentration range. Autocatalytic plume simulations revealed that pinch-off is
triggered by the appearance of accelerating flow in the plume conduit.Comment: 10 figures. Accepted for publication in Phys Rev E. See also
http://www.physics.utoronto.ca/nonlinear/papers_chemwave.htm
Measurement and Modeling of Dispersive Pulse Propagation in Drawn Wire Waveguides
An analytical model of dispersive pulse propagation in semi-infinite cylinders due to transient axially symmetric end conditions has been experimentally investigated. Specifically, the dispersive propagation of the first axially symmetric longitudinal mode in thin wire waveguides, which have ends in butt contact with longitudinal piezoelectric ultrasonic transducers, is examined. The method allows for prediction of a propagated waveform given a measured source waveform, together with the material properties of the cylinder. Alternatively, the source waveform can be extracted from measurement of the propagated waveform. The material properties required for implementation of the pulse propagation model are determined using guided wave phase velocity measurements. Hard tempered aluminum 1100 and 304 stainless steel wires, with 127, 305, and 406 micron diam., were examined. In general, the drawn wires were found to behave as transversely isotropic media
Six questions on the construction of ontologies in biomedicine
(Report assembled for the Workshop of the AMIA Working Group on Formal Biomedical Knowledge Representation in connection with AMIA Symposium, Washington DC, 2005.)
Best practices in ontology building for biomedicine have been frequently discussed in recent years. However there is a range of seemingly disparate views represented by experts in the field. These views not only reflect the different uses to which ontologies are put, but also the experiences and disciplinary background of these experts themselves. We asked six questions related to biomedical ontologies to what we believe is a representative sample of ontologists in the biomedical field and came to a number conclusions which we believe can help provide an insight into the practical problems which ontology builders face today
- …