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 $\tau= 3/2$. 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 $\tau = 1.51 \pm 0.02$, 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

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 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

First energetic neutral atom images from Polar

Energetic neutral atoms are created when energetic magnetospheric ions undergo charge exchange with cold neutral atoms in the Earth\u27s tenuous extended atmosphere (the geocorona). Since they are unaffected by the Earth\u27s magnetic field, these energetic neutrals travel away in straight line trajectories from the points of charge exchange. The remote detection of these particles provides a powerful means through which the global distribution and properties of the geocorona and ring current can be inferred. Due to its 2 × 9 RE polar orbit, the Polar spacecraft provides an excellent platform from which to observe ENAs because it spends much of its time in the polar caps which are usually free from the contaminating energetic charged particles that make observations of ENAs more difficult. In this brief report, we present the first ENA imaging results from Polar. Storm-time ENA images are presented for a northern polar cap apogee pass on August 29, 1996 and for a southern polar cap perigee pass on October 23, 1996. As well, we show with a third event (July 31, 1996) that ENA emissions can also be detected in association with individual substorm