Spinning AdS Propagators

We develop the embedding formalism to describe symmetric traceless tensors in Anti-de Sitter space. We use this formalism to construct the bulk-to-bulk propagator of massive spin J fields and check that it has the expected short distance and massless limits. We also and a split representation for the bulk-to-bulk propagator, by writing it as an integral over the boundary of the product of two bulk-to-boundary propagators. We exemplify the use of this representation with the computation of the conformal partial wave decomposition of Witten diagrams. In particular, we determine the Mellin amplitude associated to AdS graviton exchange between minimally coupled scalars of general dimension, including the regular part of the amplitude.Comment: 48 pages, 6 figure

Elastic Foundations as Heterogeneous Adventitial Boundary Condition for the Assessment of Aortic Wall and Peri-Aortic Stiffness from Dense-MRI Data Using Inverse FEM Approach

Background: The establishment of in vivo, patient-specific, and regionally resolved techniques to quantify aortic properties is key for improving risk assessment in clinical practice and scientific understanding of cardiovascular growth and remodeling. Many in vivo studies quantify vascular stiffness using Pulse Wave Velocity. This method provides an averaged measure of stiffness for the entire aorta, ignoring variations in wall stiffness and boundary conditions. Previous studies using Displacement Encoding with Stimulated Echoes Magnetic Resonance Imaging (DENSE-MRI) suggested that the infrarenal abdominal aorta (IAA) deforms heterogeneously throughout the cardiac cycle. Method: Herein, we hypothesize that the aortic wall strain heterogeneity is driven in healthy aortas by adventitial tethering to perivascular tissues that can be modeled with elastic foundation boundary conditions (EFBC) using a collection of linear-springs with a circumferential distribution of stiffness. Nine healthy-human IAAs were modeled using patient-specific imaging and displacement fields from SSFP and DENSE MRI, followed by assessment of aortic wall properties and heterogeneous EFBC parameters using inverse Finite Element Method (FEM). Results: In contrast to traction-free boundary condition, prescription of EFBC reduced the nodal displacement error by 60% and reproduced the DENSE-derived strain distribution. Estimated aortic stiffness was in agreement with previously reported experimental test data. The distribution of normalized EFBC stiffness was consistent among all patients and spatially correlated to standard peri-aortic anatomical features. Conclusion: Results suggest that EFBCs can be generalized for human adults with normal anatomy. This approach is computationally inexpensive, making it ideal for large-population clinical research and incorporation into computational cardiovascular fluid-structure analyses.https://scholarscompass.vcu.edu/gradposters/1113/thumbnail.jp

Perfusion Cell Seeding and Expansion in Dual Mechanical Stimulation Bioreactor for In Vitro Tissue Development

BACKGROUND: Engineered tissues are an exciting potential source of small diameter vascular grafts due to limited supply and patency of available alternatives. Engineered tissue vascular grafts (ETVGs) will provide physiological function that resembles native arteries and maintain the required mechanical properties as they integrate with host tissue. Mechanical stimulation during incubation encourages proper cell alignment and increases extracellular matrix deposition. The enhanced organization of the engineered tissue leads to improved compliance over synthetic alternatives without sacrificing strength and may lead to better integration in vivo. METHODS: We have developed a bioreactor that mechanically trains grafts during incubation. To test the seeding efficiency of the bioreactor, rat vascular smooth muscle cells (VSMC) were seeded onto electrospun PCL scaffolds by perfusion at various cell concentrations then incubated Page | 15 for 1 week under static conditions. We assessed gross morphology with H&E; collagen with picrosirius red; and VSMC density with DAPI. ETVGs were further evaluated with mechanical testing and scanning electron microscopy to evaluate mechanical and microstructural properties. RESULTS: Cells were successfully seeded evenly onto the luminal surface of electrospun PCL scaffolds. Cells remained viable and continued to proliferate and deposit ECM throughout incubation. CONCLUSIONS: Progress in the ETVG paradigm requires a systematic approach toward better understanding of the cause-effect interplay between implant properties, host reactions, and their modulation with controllable parameters. Future directions involve the assessment of the effects of mechanical training on growth and remodeling of engineered tissues in vitro and subsequent effects on the foreign body response post-implantation in a murine model.https://scholarscompass.vcu.edu/gradposters/1115/thumbnail.jp

Solar Neutrinos: Spin Flavour Precession and LMA

The time dependence that appears to be hinted by the data from the first 13 years of the solar neutrino Gallium experiments is viewed as resulting from a partial conversion of active neutrinos to light sterile ones through the resonant interaction between the magnetic moment of the neutrino and a varying solar field. A summary of the model and its predictions are presented for the forthcoming experiments Borexino and LENS.Comment: 6 pages, 3 figures, contribution to 12th Lomonosov Conference in Elementary Particle Physics, Moscow, Aug 24-31 (2005

NNLO predictions for Z-boson pair production at the LHC

We present a calculation of the NNLO QCD corrections to Z-boson pair production at hadron colliders, based on the N-jettiness method for the real radiation parts. We discuss the size and shape of the perturbative corrections along with their associated scale uncertainties and compare our results to recent LHC data at $\sqrt{s}=13$ TeV.Comment: 19 pages, 2 Tables, 4 figures. Version to appear in JHE

The role of leading twist operators in the Regge and Lorentzian OPE limits

We study two kinematical limits, the Regge limit and the Lorentzian OPE limit, of the four-point function of the stress-tensor multiplet in Super Yang-Mills at weak coupling. We explain how both kinematical limits are controlled by the leading twist operators. We use the known expression of the four-point function up to three loops, to extract the pomeron residue at next-to-leading order. Using this data and the known form of pomeron spin up to next-to-leading order, we predict the behaviour of the four-point function in the Regge limit at higher loops. Specifically, we determine the leading log behaviour at any loop order and the next-to-leading log at four loops. Finally, we check the consistency of our results with conformal Regge theory. This leads us to predict the behaviour around $J=1$ of the OPE coefficient of the spin $J$ leading twist operator in the OPE of two chiral primary operators.Comment: 22+14 page

Nonparametric models of financial leverage decisions

This paper investigates the properties of nonparametric decision tree models in the analysis of financial leverage decisions. This approach presents two appealing features: the relationship between leverage ratios and the explanatory variables is not predetermined but is derived according to information provided by the data, and the models respect the bounded and fractional nature of leverage ratios. The analysis shows that tree models suggest relationships between explanatory variables and the relative amount of issued debt that parametric models fail to capture. Furthermore, the significant relationships found by tree models are in most cases in accordance with the effects predicted by the pecking-order theory. The results also show that two-part tree models can accommodate better the distinct effects of explanatory variables on the decision to issue debt and on the amount of debt issued by firms that do resort to debt.Capital structure, Fractional regression, Decision trees, Two-part models

A simple rule for the evolution of fast dispersal at the edge of expanding populations

Evolution by natural selection is commonly perceived as a process that favors those that replicate faster to leave more offspring; nature, however, seem to abound with examples where organisms forgo some replicative potential to disperse faster. When does selection favor invasion of the fastest? Motivated by evolution experiments with swarming bacteria we searched for a simple rule. In experiments, a fast hyperswarmer mutant that pays a reproductive cost to make many copies of its flagellum invades a population of mono-flagellated bacteria by reaching the expanding population edge; a two-species mathematical model explains that invasion of the edge occurs only if the invasive species' expansion rate, v₂, which results from the combination of the species growth rate and its dispersal speed (but not its carrying capacity), exceeds the established species', v₁. The simple rule that we derive, v₂ > v₁, appears to be general: less favorable initial conditions, such as smaller initial sizes and longer distances to the population edge, delay but do not entirely prevent invasion. Despite intricacies of the swarming system, experimental tests agree well with model predictions suggesting that the general theory should apply to other expanding populations with trade-offs between growth and dispersal, including non-native invasive species and cancer metastases.First author draf