12,720 research outputs found
Simultaneous Heavy Ion Dissociation at Ultrarelativistic Energies
We study the simultaneous dissociation of heavy ultrarelativistic nuclei
followed by the forward-backward neutron emission in peripheral collisions at
colliders. The main contribution to this particular heavy-ion dissociation
process, which can be used as a beam luminosity monitor, is expected to be due
to the electromagnetic interaction. The Weizsacker-Williams method is extended
to the case of simultaneous excitation of collision partners which is simulated
by the RELDIS code. A contribution to the dissociation cross section due to
grazing nuclear interactions is estimated within the abrasion model and found
to be relatively small.Comment: Talk given at Bologna 2000 Conference - Structure of the Nucleus at
the Dawn of the Century, May 29 - June 3, 2000, 4 pages, 2 figure
The investigation of time dependent flame structure by ionization probes
Ionization probes were used to measure mean ionization current and frequency spectra, auto-correlations and cross-correlations in jet flames with variation in the initial Reynolds numbers and equivalence ratios. Special attention was paid to the transitional region between the burner exit plane and the plane of onset of turbulence
Topological mass mechanism and exact fields mapping
We present a class of mappings between models with topological mass mechanism
and purely topological models in arbitrary dimensions. These mappings are
established by directly mapping the fields of one model in terms of the fields
of the other model in closed expressions. These expressions provide the
mappings of their actions as well as the mappings of their propagators. For a
general class of models in which the topological model becomes the BF model the
mappings present arbitrary functions which otherwise are absent for
Chern-Simons like actions. This work generalizes the results of [1] for
arbitrary dimensions.Comment: 11 page
Geology of the Snap Lake kimberlite intrusion, Northwest Territories, Canada: field observations and their interpretation
The Cambrian (523 Ma) Snap Lake hypabyssal kimberlite intrusion, Northwest Territories, Canada, is a complex segmented diamond-bearing ore-body. Detailed geological investigations suggest that the kimberlite is a multi-phase intrusion with at least four magmatic lithofacies. In particular, olivine-rich (ORK) and olivine-poor (OPK) varieties of hypabyssal kimberlite have been identified. Key observations are that the olivine-rich lithofacieshas a strong tendency to be located where the intrusion is thickest and that there is a good correlation between intrusion thickness, olivine crystal size and crystal content. Heterogeneities in the lithofacies are attributed to variations in intrusion thickness and structural complexities. The geometry and distribution of lithofacies points to magmaticco-intrusion, and flow segregation driven by fundamental rheological differences between the two phases. We envisage that the low-viscosity OPK magma acted as a lubricant for the highly viscous ORK magma. The presenceof such low-viscosity, crystal-poor magmas may explain how crystal-laden kimberlite magmas (>60 vol.%) are able to reach the surface during kimberlite eruptions. We also document the absence of crystal settling and the development of an unusual subvertical fabric of elongate olivine crystals, which are explained by rapid degassing-induced quench crystallization of the magmas during and after intrusio
Walls, enclaves and the (counter) politics of design
This paper focuses on the political role of urban design in the transformation of urban and rural, central and peripheral, formal and informal landscapes in Israel. Based on design anthropology methodology, the political role of urban design in the production of aesthetic objects and landscapes that signify the control over individuals and communities will be explored. As this paper suggests, such a new form of political influence is hidden beneath an aesthetic and user-oriented façade, making it even more dangerous than previous more direct actions, such as gated communities separated from public space by stone walls. The paper’s interdisciplinary approach that is rooted in anthropology, design, architecture and politics will also point out some similarities between specific sites that are often considered different, namely Tel Aviv’s global and privatized gated communities on the one hand and the unrecognized Bedouin villages in the peripheral Negev region on the other. It will be argued that these similarities are the product of the politics of militarization, privatization and social fragmentation that are translated into urban design practices from ‘above’ via state and municipal planning policy as well as formal design, and from ‘below’ through informal and often unauthorized construction initiated by marginalized communities
Electromigration in thin tunnel junctions with ferromagnetic/nonmagnetic: nanoconstrictions, local heating, and direct and wind forces
Current Induced Resistance Switching (CIS) was recently observed in thin
tunnel junctions with ferromagnetic (FM) electrodes \emph{i.e} FM/I/FM. This
effect was attributed to electromigration of metallic atoms in
nanoconstrictions in the insulating barrier (I). Here we study how the CIS
effect is influenced by a thin non-magnetic (NM) Ta layer, deposited just below
the AlO insulating barrier in tunnel junctions of the type FM/NM/I/FM
(FM=CoFe). Enhanced resistance switching occurs with increasing maximum applied
current (\Imax), until a plateau of constant CIS is reached for \Imax\sim65
mA (CIS60%) and above. However, such high electrical currents also lead
to a large (9%) irreversible resistance decrease, indicating barrier
degradation. Anomalous voltage-current characteristics with negative derivative
were also observed near \pm\Imax and this effect is here attributed to
heating in the tunnel junction. One observes that the current direction for
which resistance switches in FM/NM/I/FM (clockwise) is opposite to that of
FM/I/FM tunnel junctions (anti-clockwise). This effect will be discussed in
terms of a competition between the electromigration contributions due to the so
called direct and wind forces. It will be shown that the direct force is likely
to dominate electromigration in the Ta (NM) layers, while the wind contribution
likely dominates in the CoFe (FM) layers
Multi-Objective Genetic Programming for Feature Extraction and Data Visualization
Feature extraction transforms high dimensional
data into a new subspace of lower dimensionalitywhile keeping
the classification accuracy. Traditional algorithms do not
consider the multi-objective nature of this task. Data transformations
should improve the classification performance
on the new subspace, as well as to facilitate data visualization,
which has attracted increasing attention in recent years.
Moreover, new challenges arising in data mining, such as
the need to deal with imbalanced data sets call for new algorithms
capable of handling this type of data. This paper
presents a Pareto-basedmulti-objective genetic programming
algorithm for feature extraction and data visualization. The
algorithm is designed to obtain data transformations that optimize
the classification and visualization performance both
on balanced and imbalanced data. Six classification and visualization
measures are identified as objectives to be optimized
by the multi-objective algorithm. The algorithm is
evaluated and compared to 11 well-known feature extraction
methods, and to the performance on the original high
dimensional data. Experimental results on 22 balanced and
20 imbalanced data sets show that it performs very well on
both types of data, which is its significant advantage over
existing feature extraction algorithms
X-ray Properties of Pre--Main-Sequence Stars in the Orion Nebula Cluster with Known Rotation Periods
We re-analyze all archival Chandra/ACIS observations of the Orion Nebula
Cluster (ONC) to study the X-ray properties of a large sample of
pre--main-sequence (PMS) stars with optically determined rotation periods. Our
goal is to elucidate the origins of X-rays in PMS stars by seeking out
connections between the X-rays and the mechanisms most likely driving their
production--rotation and accretion. In our sample X-ray luminosity is
significantly correlated with stellar rotation, in the sense of decreasing
Lx/Lbol with more rapid rotation, suggesting that these stars are in the
"super-saturated" regime of the rotation-activity relationship. However, we
also find that stars with optical rotation periods are significantly biased to
high Lx. This is not the result of magnitude bias in the optical
rotation-period sample but rather to the diminishingly small amplitude of
optical variations in stars with low Lx. Evidently, there exists in the ONC a
population of stars whose rotation periods are unknown and that possess lower
average X-ray luminosities than those of stars with known rotation periods.
These stars may sample the linear regime of the rotation-activity relationship.
Accretion also manifests itself in X-rays, though in a somewhat
counterintuitive fashion: While stars with spectroscopic signatures of
accretion show harder X-ray spectra than non-accretors, they show lower X-ray
luminosities and no enhancement of X-ray variability. We interpret these
findings in terms of a common origin for the X-ray emission observed from both
accreting and non-accreting stars, with the X-rays from accreting stars simply
being attenuated by magnetospheric accretion columns. This suggests that X-rays
from PMS stars have their origins primarily in chromospheres, not accretion.Comment: Accepted by the Astronomical Journal. 43 pages, 16 figure
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