Landau-Ginzburg Vacua of String, M- and F-Theory at c=12

Theories in more than ten dimensions play an important role in understanding nonperturbative aspects of string theory. Consistent compactifications of such theories can be constructed via Calabi-Yau fourfolds. These models can be analyzed particularly efficiently in the Landau-Ginzburg phase of the linear sigma model, when available. In the present paper we focus on those sigma models which have both a Landau-Ginzburg phase and a geometric phase described by hypersurfaces in weighted projective five-space. We describe some of the pertinent properties of these models, such as the cohomology, the connectivity of the resulting moduli space, and mirror symmetry among the 1,100,055 configurations which we have constructed.Comment: LaTeX, 33 pages, 10 PostScript figures using epsfig and psfi

Overestimation of Vitamin a Supplementation Coverage from District Tally Sheets Demonstrates Importance of Population-Based Surveys for Program Improvement: Lessons from Tanzania.

Tanzania has conducted a national twice-yearly Vitamin A supplementation (VAS) campaign since 2001. Administrative coverage rates based on tally sheets consistently report >90% coverage; however the accuracy of these rates are uncertain due to potential errors in tally sheets and their aggregation, incomplete or inaccurate reporting from distribution sites, and underestimating the target population. The post event coverage survey in Mainland Tanzania sought to validate tally-sheet based national coverage estimates of VAS and deworming for the June 2010 mass distribution round, and to characterize children missed by the national campaign. WHO/EPI randomized cross-sectional cluster sampling methodology was adapted for this study, using 30 clusters by 40 individuals (n = 1200), in addition to key informant interviews. Households with children 6-59 months of age were included in the study (12-59 months for deworming analysis). Chi-squared tests and logistic regression analysis were used to test differences between children reached and not reached by VAS. Data was collected within six weeks of the June 2010 round. A total of 1203 children, 58 health workers, 30 village leaders and 45 community health workers were sampled. Preschool VAS coverage was 65% (95% CI: 62.7-68.1), approximately 30% lower than tally-sheet coverage estimates. Factors associated with not receiving VAS were urban residence [OR = 3.31; p = 0.01], caretakers who did not hear about the campaign [OR = 48.7; p<0.001], and Muslim households [OR<3.25; p<0.01]. There were no significant differences in VAS coverage by child sex or age, or maternal age or education. Coverage estimation for vitamin A supplementation programs is one of most powerful indicators of program success. National VAS coverage based on a tally-sheet system overestimated VAS coverage by ∼30%. There is a need for representative population-based coverage surveys to complement and validate tally-sheet estimates

Three-dimensional flow characteristics in ventricular assist devices: Impact of valve design and operating conditions

ObjectiveThe use of paracorporeal ventricular assist devices has become a well-established procedure for patients with cardiogenic shock. However, implantation of ventricular assist devices is often associated with severe complications, such as thrombosis inside the ventricular assist device and subsequent embolic events. It was the purpose of this study to use flow-sensitive 4-dimensional magnetic resonance imaging for a detailed analysis of the 3-dimensional (3D) flow dynamics inside a clinical routine ventricular assist device and to study the effect of different system adjustments and a new valve design on flow patterns.MethodsA routinely used clinical paracorporeal ventricular assist device was integrated into a magnetic resonance-compatible mock loop. Flow-sensitive 3D magnetic resonance imaging was performed to measure time-resolved 3-directional flow velocities (spatial resolution ∼ 1.2 mm, temporal resolution = 42.4 ms) in the entire device under ideal conditions (full fill, full empty, ejection fraction = 88%), insufficient filling (ejection fraction = 81%), and insufficient emptying (ejection fraction = 67%) of the pump chamber. In addition, a new valve design was evaluated. Flexible control and monitoring of pressures at inlet and outlet were used to generate realistic boundary conditions.ResultsFlow pattern changes for different operating conditions were clearly identified and included reduced velocities during systolic outflow for impaired filling (78% reduction in pump flow compared with optimal operating conditions) and impaired clearing of the pump chamber for insufficient emptying (52% reduction). For all operating conditions, 3D visualization revealed vortex flow inside the ventricular assist device at typical locations of thrombus formation near the valve systems. Most noticeably, the new valve design provided similar global ventricular assist device function (pump flow 3.6 L/min), but vortex formation was eliminated.ConclusionsThe results of this study provide insight into the mechanisms underlying possible thrombus formation inside a ventricular assist device and the effect of different system adjustments. The presented methods may permit the optimization of future ventricular assist device systems with respect to optimal flow conditions

Heterotic Gauge Structure of Type II K3 Fibrations

We show that certain classes of K3 fibered Calabi-Yau manifolds derive from orbifolds of global products of K3 surfaces and particular types of curves. This observation explains why the gauge groups of the heterotic duals are determined by the structure of a single K3 surface and provides the dual heterotic picture of conifold transitions between K3 fibrations. Abstracting our construction from the special case of K3 hypersurfaces to general K3 manifolds with an appropriate automorphism, we show how to construct Calabi-Yau threefold duals for heterotic theories with arbitrary gauge groups. This generalization reveals that the previous limit on the Euler number of Calabi-Yau manifolds is an artifact of the restriction to the framework of hypersurfaces.Comment: 15 pages, 3 eps figure

Unification of M- and F- Theory Calabi-Yau Fourfold Vacua

We consider splitting type phase transitions between Calabi-Yau fourfolds. These transitions generalize previously known types of conifold transitions between threefolds. Similar to conifold configurations the singular varieties mediating the transitions between fourfolds connect moduli spaces of different dimensions, describing ground states in M- and F-theory with different numbers of massless modes as well as different numbers of cycles to wrap various p-branes around. The web of Calabi-Yau fourfolds obtained in this way contains the class of all complete intersection manifolds embedded in products of ordinary projective spaces, but extends also to weighted configurations. It follows from this that for some of the fourfold transitions vacua with vanishing superpotential are connected to ground states with nonzero superpotential.Comment: plain TeX, 22p

Landau-Ginzburg String Vacua

We investigate a class of (2,2) supersymmetric string vacua which may be represented as Landau--Ginzburg theories with a quasihomogeneous potential which has an isolated singularity at the origin. There are at least three thousand distinct models in this class. All vacua of this type lead to Euler numbers which lie in the range $-960 \leq \chi \leq 960$. The Euler characteristics do not pair up completely hence the space of Landau--Ginzburg ground states is not mirror symmetric even though it exhibits a high degree of symmetry. We discuss in some detail the relation between Landau--Ginzburg models and Calabi--Yau manifolds and describe a subtlety regarding Landau--Ginzburg potentials with an arbitrary number of fields. We also show that the use of topological identities makes it possible to relate Landau-Ginzburg theories to types of Calabi-Yau manifolds for which the usual Landau-Ginzburg framework does not apply.Comment: 92p

Constructing Indices of Rural Living Standards in Northwestern Bangladesh

This study aimed to construct indices of living standards in rural Bangladesh that could be useful to study health outcomes or identify target populations for poverty-alleviation programmes. The indices were constructed using principal component analysis of data on household assets and house construction materials. Their robustness and use was tested and found to be internally consistent and correlated with maternal and infant health, nutritional and demographic indicators, and infant mortality. Indices derived from 9 or 10 household asset variables performed well; little was gained by adding more variables but problems emerged if fewer variables were used. A ranking of the most informative assets from this rural, South Asian context is provided. Living standards consistently and significantly improved over the six-year study period. It is concluded that simple household socioeconomic data, collected under field conditions, can be used for constructing reliable and useful indices of living standards in rural South Asian communities that can assist in the assessment of health, quality of life, and capabilities of households and their members