13,518 research outputs found
T-Branes and Geometry
T-branes are a non-abelian generalization of intersecting branes in which the
matrix of normal deformations is nilpotent along some subspace. In this paper
we study the geometric remnant of this open string data for six-dimensional
F-theory vacua. We show that in the dual M-theory / IIA compactification on a
smooth Calabi-Yau threefold X, the geometric remnant of T-brane data translates
to periods of the three-form potential valued in the intermediate Jacobian of
X. Starting from a smoothing of a singular Calabi-Yau, we show how to track
this data in singular limits using the theory of limiting mixed Hodge
structures, which in turn directly points to an emergent Hitchin-like system
coupled to defects. We argue that the physical data of an F-theory
compactification on a singular threefold involves specifying both a geometry as
well as the remnant of three-form potential moduli and flux which is localized
on the discriminant. We give examples of T-branes in compact F-theory models
with heterotic duals, and comment on the extension of our results to
four-dimensional vacua.Comment: v2: 80 pages, 2 figures, clarifications and references added, typos
correcte
Top-Quark Mass and Bottom-Quark Decay
The possibility of a long B-meson lifetime is explored, in which case the weak mixing angles θ_2 and θ_3 are quite small. This allows the derivation of a lower bound on the top-quark mass as a function of the B-meson lifetime, by comparison of the short-distance prediction for the CP-nonconservation parameter ε with its experimental value. The bound is significant for τ_B>4×10^(-13) s
Critical branching processes in digital memcomputing machines
Memcomputing is a novel computing paradigm that employs time non-locality
(memory) to solve combinatorial optimization problems. It can be realized in
practice by means of non-linear dynamical systems whose point attractors
represent the solutions of the original problem. It has been previously shown
that during the solution search digital memcomputing machines go through a
transient phase of avalanches (instantons) that promote dynamical long-range
order. By employing mean-field arguments we predict that the distribution of
the avalanche sizes follows a Borel distribution typical of critical branching
processes with exponent . We corroborate this analysis by solving
various random 3-SAT instances of the Boolean satisfiability problem. The
numerical results indicate a power-law distribution with exponent , in very good agreement with the mean-field analysis. This indicates
that memcomputing machines self-tune to a critical state in which avalanches
are characterized by a branching process, and that this state persists across
the majority of their evolution.Comment: 5 pages, 3 figure
A Library They Deserve: The Baltimore Elementary and Middle School Library Project
The Harry and Jeanette Weinberg Foundation has partnered with Baltimore Education Research Consortium (BERC) to complete a series of reports examining the implementation and impact of the Baltimore Elementary and Middle School Library Project (Library Project). This report on the first year of the project examines the experiences of the principals, librarians, teachers, and students at the three schools that received new libraries. Also, teachers' and students' perceptions of their school library were compared across the Library Project and comparison schools. Findings of the report include: (1) all three schools with a new library viewed them as inviting, attractive, and well-resourced spaces; (2) a knowledgeable, skilled, and motivated library staff is essential to maximizing the potential of these new libraries; (3) librarians and teachers need additional professional development to best integrate the library technology into instruction; and (4) the community partnerships initiated through this project are adding significant resources to the education of students in these schools. The implications for the school district and future efforts are discussed
Deter War, Not Attacks Against Space Systems
The “Space Deterrence: The Delicate Balance of Risk” study by the Eisenhower Center for Space and Defense Studies has much merit to it. First, to undertake the task at all is praiseworthy given the enormity and importance of the topic. Second, the study contains many sensible points, ranging from the uncertain nature of deterrence to measures needed to physically protect space systems that policy makers and students of strategy would do well to note. But as noteworthy as the Space Deterrence study is, there are two wider points to consider that are not found in its pages. The omission of these points are not necessarily the fault of the authors of the study, given the parameters set out by the study’s sponsor, but they are worth pondering nonetheless
T-Branes at the Limits of Geometry
Singular limits of 6D F-theory compactifications are often captured by
T-branes, namely a non-abelian configuration of intersecting 7-branes with a
nilpotent matrix of normal deformations. The long distance approximation of
such 7-branes is a Hitchin-like system in which simple and irregular poles
emerge at marked points of the geometry. When multiple matter fields localize
at the same point in the geometry, the associated Higgs field can exhibit
irregular behavior, namely poles of order greater than one. This provides a
geometric mechanism to engineer wild Higgs bundles. Physical constraints such
as anomaly cancellation and consistent coupling to gravity also limit the order
of such poles. Using this geometric formulation, we unify seemingly different
wild Hitchin systems in a single framework in which orders of poles become
adjustable parameters dictated by tuning gauge singlet moduli of the F-theory
model.Comment: v2: 65 pages, 6 figures, clarifications adde
The Retention Problem: An Analysis of Enrolment Attrition at a Canadian College
A survey of returning and non-returning students at Scarborough College was undertaken to investigate the problem of declining retention rates at Ontario Universities. This study describes the opportunities available to and chosen by non-returning students, and then seeks to evaluate various hypotheses that have been proposed to account for enrolment attrition. Consistent with prior research, there was no evidence found to support the claim that students are being forced out of university by an inability to afford the costs of their education. Instead, the results suggested that enrolment attrition is produced by a combination of an attenuation of the student's commitment to obtain the degree and the student's awareness of and access to viable alternatives. As a result of a pervasive sense of uncertainty about the market value of a university degree, many students are willing to assay other opportunities. Specifically, those students who went to work appear to have been "pulled out" of university by the availability of a credible job. In addition, the contingencies affecting the commitment of the returning students and the students who transfer to other educational institutions are discussed. The conclusion addresses the relationship between financial circumstances and access to university, and raises questions about future enrolment patterns.Une enquête sur les étudiants qui continuent à fréquenter et ceux qui laissent le Collège de Scarborough fut entreprise dans le but d'étudier le problème des taux de rétention en déclin dans les universités de l'Ontario. La présente étude s'arrête à analyser les raisons d'abandon des étudiants et cherche ensuite à vérifier diverses hypothèses avancées pour expliquer la baisse des inscriptions. En conformité avec des recherches antérieures, aucune évidence n 'a pu être trouvée pour supporter l'idée que les étudiants soient forcés d'inter-rompre leurs études universitaires à cause d'une incapacité à supporter les coûts de l'en-seignement. Au contraire, les résultats de la recherche suggère que la baisse des inscriptions provient plutôt d'un manque d'intérêt pour l'obtention d'un diplôme de même que d'une meilleure connaissance de d'autres possibilités offertes sur le marché du travail. Etant donné le degré d'incertitude planant sur la valeur marchande d'un diplôme universitaire, de nombreux étudiants sont prêts à explorer d'autres avenues. Plus spécifiquement, les étudiants qui ont abandonné leurs études semblent avoir été attirés par la disponibilité d'un emploi satisfaisant. L'auteur discute également des circonstances entourant les étudiants qui retournent et ceux qui décident de transférer dans d'autres institutions d'enseignement. Les conclusions de cette étude touchent aussi au problème de l'accessi-bilité à l'université et des dépenses encourues; finalement, certaines questions sont posées au sujet des tendances futures des inscriptions universitaires
The JPL telerobot operator control station. Part 2: Software
The Operator Control Station of the Jet Propulsion Laboratory (JPL)/NASA Telerobot Demonstrator System provides the man-machine interface between the operator and the system. It provides all the hardware and software for accepting human input for the direct and indirect (supervised) manipulation of the robot arms and tools for task execution. Hardware and software are also provided for the display and feedback of information and control data for the operator's consumption and interaction with the task being executed. The software design of the operator control system is discussed
Discontinuous Molecular Dynamics for Rigid Bodies: Applications
Event-driven molecular dynamics simulations are carried out on two rigid body
systems which differ in the symmetry of their molecular mass distributions.
First, simulations of methane in which the molecules interact via discontinuous
potentials are compared with simulations in which the molecules interact
through standard continuous Lennard-Jones potentials. It is shown that under
similar conditions of temperature and pressure, the rigid discontinuous
molecular dynamics method reproduces the essential dynamical and structural
features found in continuous-potential simulations at both gas and liquid
densities. Moreover, the discontinuous molecular dynamics approach is
demonstrated to be between 2 to 100 times more efficient than the standard
molecular dynamics method depending on the specific conditions of the
simulation. The rigid discontinuous molecular dynamics method is also applied
to a discontinuous-potential model of a liquid composed of rigid benzene
molecules, and equilibrium and dynamical properties are shown to be in
qualitative agreement with more detailed continuous-potential models of
benzene. Qualitative differences in the dynamics of the two models are related
to the relatively crude treatment of variations in the repulsive interactions
as one benzene molecule rotates by another.Comment: 14 pages, double column revte
The discovery of trapped energetic electrons in the outer cusp
We report on the POLAR/CEPPAD discovery of a trapped, 60°\u3cθ\u3c120° pitch angle electron population in the outer cusp (7−9+ Re), whose energetic electron component extends from below 30 keV to ∼2 MeV. Because the time variability in the outer cusp precludes mapping with POLAR, we have carried out test particle simulations using the Tsyganenko 1996 model (T96) to demonstrate the trapping of these energy electrons in the outer cusp region and the resonant frequencies of its trapped motion. We discuss the boundaries and regions of the cusp trap and show that it is analogous to the dipole trap. We show that the phase space densities observed there are equal or greater than the phase space densities observed in the radiation belts at constant magnetic moment, thus allowing the possibility of diffusive filling of the radiation belts from the cus
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