63,043 research outputs found
Don't Panic! Closed String Tachyons in ALE Spacetimes
We consider closed string tachyons localized at the fixed points of
noncompact nonsupersymmetric orbifolds. We argue that tachyon condensation
drives these orbifolds to flat space or supersymmetric ALE spaces. The decay
proceeds via an expanding shell of dilaton gradients and curvature which
interpolates between two regions of distinct angular geometry. The string
coupling remains weak throughout. For small tachyon VEVs, evidence comes from
quiver theories on D-branes probes, in which deformations by twisted couplings
smoothly connect non-supersymmetric orbifolds to supersymmetric orbifolds of
reduced order. For large tachyon VEVs, evidence comes from worldsheet RG flow
and spacetime gravity. For \IC^2/\IZ_n, we exhibit infinite sequences of
transitions producing SUSY ALE spaces via twisted closed string condensation
from non-supersymmetric ALE spaces. In a -dual description this provides a
mechanism for creating NS5-branes via {\it closed} string tachyon condensation
similar to the creation of D-branes via {\it open} string tachyon condensation.
We also apply our results to recent duality conjectures involving fluxbranes
and the type 0 string.Comment: 48 pages, harvmac big; 13 figures. v2: added references; comment on
gravity regime added to section 5.2. v3: added reference
When Learning Counts: Rethinking Licenses for School Leaders
Recommends restructuring state licensing systems to focus on the skills and knowledge leaders need to improve learning, and better aligning licenses with the current job demands on principals
Searching a biomedical bibliographic database from the Ukraine: the Panteleimon database
The Panteleimon database is available via the Internet and is a public access, database, capable of being searched in English, Russian and Ukrainian, covering medical, pharmaceutical, and chemical publications, published in he Ukraine and Russia from 1998. Describes the formulation of a search strategy for the Panteleimon database, for the identification of citations to randomized controlled trials (RCTs), and the comparison of the search results with records included in the Cochrane Library's Cochrane Central Register of Controlled Trials (CENTRAL) database, to evaluate how comprehensive the coverage of the CENTRAL database is for the literature of the Ukraine. The results indicated that Panteleimon is an easily accessible bibliographic database offering easy access to the Ukrainian biomedical literature. The English language retrieval functions picked up most of the reports of RCTs/CCTs (91 per cent precision but the lower recall of 55 per cent indicates the need to search using Russian and Ukrainian terms for completeness. The overall precision of 26 per cent compares favourably with a search for RCTs in EMBASE, carried out by the UK Cochrane Centre, where 70,000 reports of RCTs were identified from 300,000 records down-loaded (precision 23 per cent). (Quotes from original text
The Ultimate Halo Mass in a LCDM Universe
In the far future of an accelerating LCDM cosmology, the cosmic web of
large-scale structure consists of a set of increasingly isolated halos in
dynamical equilibrium. We examine the approach of collisionless dark matter to
hydrostatic equilibrium using a large N-body simulation evolved to scale factor
a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h
Gyr into the future for a concordance model universe (Omega_m ~ 0.3,
Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized
at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically
active accretion region -- simplifies at a > 10 a_eq when these surfaces merge
to create a single zero-velocity surface, clearly defining the halo outer
boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed
physical size encompassing a mean interior density ~ 5 times the critical
density, similar to the turnaround value in a classical Einstein-deSitter
model. We relate mhalo to other scales currently used to define halo mass
(m200, mvir, m180b) and find that m200 is approximately half of the total
asymptotic cluster mass, while m180b follows the evolution of the inner zero
velocity surface for a < 2 but becomes much larger than the total bound mass
for a > 3. The radial density profile of all bound halo material is well fit by
a truncated Hernquist profile. An NFW profile provides a somewhat better fit
interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter
Measurement of interstage fluid-annulus dynamical properties
The work described in this paper is part of an Electric Power Research Institute sponsored effort to improve rotor vibrational performance on power plant feed water pumps. A major objective of this effort is to reduce vibration levels by devising inter-stage sealing configurations with optimized damping capacity, realizing that the typical multi-stage centrifugal pump has several ore inter-stage fluid annuli than it has journal bearings. Also, the fluid annuli are distributed between the journal bearings where vibration levels are highest and can therefore be 'exercised' more as dampers than can the bearings. Described in this paper is a test apparatus which has been built to experimentally determine fluid-annulus dynamical coefficients for various configurations of inter-stage sealing geometry
Integrated maneuvering and life support system simulation Final report
Integrated maneuvering and life support system simulatio
Half-Twisted Correlators from the Coulomb Branch
We compute correlators of chiral operators in half-twisted (0,2)
supersymmetric gauged linear sigma models. Our results give simple algebraic
formulas for a (0,2) generalization of genus zero Gromov-Witten invariants of
compact toric varieties. We derive compact expressions for deformed quantum
cohomology relations and apply our general method to several examples.Comment: 21 pages, LaTex; typos corrected; some discussion adde
GLSMs for non-Kahler Geometries
We identify a simple mechanism by which H-flux satisfying the modified
Bianchi identity arises in garden-variety (0,2) gauged linear sigma models.
Taking suitable limits leads to effective gauged linear sigma models with
Green-Schwarz anomaly cancellation. We test the quantum-consistency of a class
of such effective theories by constructing an off-shell superconformal algebra,
providing evidence that these models run to good CFTs in the deep IR.Comment: 37 pages, Minor updates for v
Leaving College: Why Students Withdrew from a University
The purpose of this study was to determine the reasons why students withdrew during a semester from a mid-sized, comprehensive university located in the Midwest. Six hundred forty-five students were asked to complete the ACT Withdrawing/Non-returning Student Survey during the 1992-93 academic year and summer semester. Three hundred sixty-five completed surveys were returned for a 57% response rate.
Respondents indicated many different reasons for leaving which varied by year in school and whether or not the respondent was a graduate or undergraduate student. There was no typical withdrawing student and there were many reasons students withdrew over which the university has little or no control. The report concludes with a discussion of Vincent Tinto\u27s (1993) ideas concerning institutional departure.
The retention and persistence of students in higher education has been the focus of serious intellectual inquiry for many years. Various concepts of institutional departure, persistence and models for programmatic interventions to reduce departure have been developed. (For example, see Pascarella & Terenzini, 1991; Stage & Rushin, 1993; Steele, Kennedy, & Gordon, 1993; Tinto, 1993; Wolfe, 1993.) The purpose of this study was to focus on one aspect of student attrition, and. to investigate the reasons and general trends as . to why students withdrew during a semester from a midsized comprehensive university located in the Midwest. This information could then be used to guide institutional action
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