15,087 research outputs found
Plio-Pleistocene time-averaged field in southern Patagonia recorded in lava flows
Paleomagnetic directions were obtained from stepwise alternating-field or thermal demagnetization of 53 lava flows from southern Patagonia (latitudes 49.5°-52.1 °S) that include the Pali-Aike volcanic field and the Meseta Viscachas plateau lavas. In addition to previous Miocene-late Quaternary ages of these flows, 40Ar/39Ar dates spanning from 0.1 to 15.4 Ma were obtained for 17 of the sites. All except one of the magnetic polarities coincide with the expected polarities of the magnetic polarity timescale [Cande and Kent, 1995] for the obtained 40Ar/39Ar ages. The mean direction from 33 sites (eliminating sites <4 Ma) that pass a selection criteria of α95 â€5° is Dec = 358.7°,Inc = - 68.2°, α95 = 3.5°, a value that coincides within the statistical uncertainty with the direction of the geocentric axial dipole for that area (Inc = - 68.1°). Likewise, the mean virtual geomagnetic pole (VGP) coincides within the statistical uncertainty with the geographic North Pole. The secular variation described by the VGP angular standard deviation for these sites is 17.1°, a value expected for that latitude according to Model G of paleosecular variation [McFadden et al., 1988]. The characteristics of the data presented are optimum for time-averaged field (TAF) studies because of the good age control and good quality of the paleomagnetic data: (1) primary components of magnetization were obtained using principal component analysis [Kirschvink, 1980] from at least five points and maximum angular deviation â€5°, (2) site means were calculated with Fisher statistics using at least three samples, and (c) 38 of the 53 flows had α95 †5°. No results (five sites) or high α95 values (â€5°) were obtained primarily from sites affected by lightning.Fil: Mejia, V.. University of Florida; Estados UnidosFil: Opdyke, N. D.. University of Florida; Estados UnidosFil: Vilas, Juan Francisco A.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de GeologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de Geociencias BĂĄsicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias BĂĄsicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Singer, B. S.. University of Wisconsin; Estados UnidosFil: Stoner, J. S.. State University of Colorado at Boulder; Estados Unido
The role of social cognition in decision making
Successful decision making in a social setting depends on our ability to understand the intentions, emotions and beliefs of others. The mirror system allows us to understand other people's motor actions and action intentions. âEmpathyâ allows us to understand and share emotions and sensations with others. âTheory of mindâ allows us to understand more abstract concepts such as beliefs or wishes in others. In all these cases, evidence has accumulated that we use the specific neural networks engaged in processing mental states in ourselves to understand the same mental states in others. However, the magnitude of the brain activity in these shared networks is modulated by contextual appraisal of the situation or the other person. An important feature of decision making in a social setting concerns the interaction of reason and emotion. We consider four domains where such interactions occur: our sense of fairness, altruistic punishment, trust and framing effects. In these cases, social motivations and emotions compete with each other, while higher-level control processes modulate the interactions of these low-level biases
Mining Missing Hyperlinks from Human Navigation Traces: A Case Study of Wikipedia
Hyperlinks are an essential feature of the World Wide Web. They are
especially important for online encyclopedias such as Wikipedia: an article can
often only be understood in the context of related articles, and hyperlinks
make it easy to explore this context. But important links are often missing,
and several methods have been proposed to alleviate this problem by learning a
linking model based on the structure of the existing links. Here we propose a
novel approach to identifying missing links in Wikipedia. We build on the fact
that the ultimate purpose of Wikipedia links is to aid navigation. Rather than
merely suggesting new links that are in tune with the structure of existing
links, our method finds missing links that would immediately enhance
Wikipedia's navigability. We leverage data sets of navigation paths collected
through a Wikipedia-based human-computation game in which users must find a
short path from a start to a target article by only clicking links encountered
along the way. We harness human navigational traces to identify a set of
candidates for missing links and then rank these candidates. Experiments show
that our procedure identifies missing links of high quality
The Algebras of Large N Matrix Mechanics
Extending early work, we formulate the large N matrix mechanics of general
bosonic, fermionic and supersymmetric matrix models, including Matrix theory:
The Hamiltonian framework of large N matrix mechanics provides a natural
setting in which to study the algebras of the large N limit, including
(reduced) Lie algebras, (reduced) supersymmetry algebras and free algebras. We
find in particular a broad array of new free algebras which we call symmetric
Cuntz algebras, interacting symmetric Cuntz algebras, symmetric
Bose/Fermi/Cuntz algebras and symmetric Cuntz superalgebras, and we discuss the
role of these algebras in solving the large N theory. Most important, the
interacting Cuntz algebras are associated to a set of new (hidden) local
quantities which are generically conserved only at large N. A number of other
new large N phenomena are also observed, including the intrinsic nonlocality of
the (reduced) trace class operators of the theory and a closely related large N
field identification phenomenon which is associated to another set (this time
nonlocal) of new conserved quantities at large N.Comment: 70 pages, expanded historical remark
Gauge-Invariant Coordinates on Gauge-Theory Orbit Space
A gauge-invariant field is found which describes physical configurations,
i.e. gauge orbits, of non-Abelian gauge theories. This is accomplished with
non-Abelian generalizations of the Poincare'-Hodge formula for one-forms. In a
particular sense, the new field is dual to the gauge field. Using this field as
a coordinate, the metric and intrinsic curvature are discussed for Yang-Mills
orbit space for the (2+1)- and (3+1)-dimensional cases. The sectional, Ricci
and scalar curvatures are all formally non-negative. An expression for the new
field in terms of the Yang-Mills connection is found in 2+1 dimensions. The
measure on Schroedinger wave functionals is found in both 2+1 and 3+1
dimensions; in the former case, it resembles Karabali, Kim and Nair's measure.
We briefly discuss the form of the Hamiltonian in terms of the dual field and
comment on how this is relevant to the mass gap for both the (2+1)- and
(3+1)-dimensional cases.Comment: Typos corrected, more about the non-Abelian decomposition and inner
products, more discussion of the mass gap in 3+1 dimensions. Now 23 page
Epigenetic Chromatin Silencing: Bistability and Front Propagation
The role of post-translational modification of histones in eukaryotic gene
regulation is well recognized. Epigenetic silencing of genes via heritable
chromatin modifications plays a major role in cell fate specification in higher
organisms. We formulate a coarse-grained model of chromatin silencing in yeast
and study the conditions under which the system becomes bistable, allowing for
different epigenetic states. We also study the dynamics of the boundary between
the two locally stable states of chromatin: silenced and unsilenced. The model
could be of use in guiding the discussion on chromatin silencing in general. In
the context of silencing in budding yeast, it helps us understand the phenotype
of various mutants, some of which may be non-trivial to see without the help of
a mathematical model. One such example is a mutation that reduces the rate of
background acetylation of particular histone side-chains that competes with the
deacetylation by Sir2p. The resulting negative feedback due to a Sir protein
depletion effect gives rise to interesting counter-intuitive consequences. Our
mathematical analysis brings forth the different dynamical behaviors possible
within the same molecular model and guides the formulation of more refined
hypotheses that could be addressed experimentally.Comment: 19 pages, 5 figure
The Quasi-Molecular Stage of Ternary Fission
We developed a three-center phenomenological model,able to explain
qualitatively the recently obtained experimental results concerning the
quasimolecular stage of a light-particle accompanied fission process. It was
derived from the liquid drop model under the assumption that the aligned
configuration, with the emitted particle between the light and heavy fragment,
is reached by increasing continuously the separation distance, while the radii
of the heavy fragment and of the light particle are kept constant. In such a
way,a new minimum of a short-lived molecular state appears in the deformation
energy at a separation distance very close to the touching point. This minimum
allows the existence of a short-lived quasi-molecular state, decaying into the
three final fragments.The influence of the shell effects is discussed. The
half-lives of some quasimolecular states which could be formed in the Be
and C accompanied fission of Cf are roughly estimated to be the
order of 1 ns, and 1 ms, respectively.Comment: 12 pages, 6 epsf, uses ws-p8-50x6-00.cl
Magnetic friction due to vortex fluctuation
We use Monte Carlo and molecular dynamics simulation to study a magnetic
tip-sample interaction. Our interest is to understand the mechanism of heat
dissipation when the forces involved in the system are magnetic in essence. We
consider a magnetic crystalline substrate composed of several layers
interacting magnetically with a tip. The set is put thermally in equilibrium at
temperature T by using a numerical Monte Carlo technique. By using that
configuration we study its dynamical evolution by integrating numerically the
equations of motion. Our results suggests that the heat dissipation in this
system is closed related to the appearing of vortices in the sample.Comment: 6 pages, 41 figure
The strong-CP question in SU(3)_c X SU(3)_L X U(1)_N models
We analyze two recent models based on the gauge group
SU(3)SU(3)U(1) where each generation is not
anomaly-free, but anomaly cancels when three generations are taken into
account. We show that the most general Yukawa couplings of these models admit
of a Peccei-Quinn symmetry. This symmetry can be extended to the entire
Lagrangian by using extra fields in a very elegant way so that the resulting
axion can be made invisible.Comment: Latex, 8 pages, no figure
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