A simple derivation of the Overlap Dirac Operator

We derive the vector-like four dimensional overlap Dirac operator starting from a five dimensional Dirac action in the presence of a delta-function space-time defect. The effective operator is obtained by first integrating out all the fermionic modes in the fixed gauge background, and then identifying the contribution from the localized modes as the determinant of an operator in one dimension less. We define physically relevant degrees of freedom on the defect by introducing an auxiliary defect-bound fermion field and integrating out the original five dimensional bulk field.Comment: 9 pages, LaTe

Dynamic, viscoelasticity-driven shape change of elastomer bilayers

Thin bilayers made of elastic sheets with different strain recoveries can be used for dynamic shape morphing through ambient stimuli, such as temperature, mass diffusion, and light. As a fundamentally different approach to designing temporal shape change, constituent polymer molecular features (rather than external fields) are leveraged, specifically the viscoelasticity of gelatin bilayers, to achieve dynamic three-dimensional (3D) curls and helical twists. After stretching and releasing, the acquired 3D shape recovers its original flat shape on a timescale originating from the polymer viscoelasticity. The bilayer time-dependent curvature can be accurately predicted from hyperelastic and viscoelastic functions using finite element analysis (FEA). FEA reveals the nonlinear shape dynamics in space and time in quantitative agreement with experiments. The findings present a new frontier in dynamic biomimetic shape-morphing by exploiting intrinsic material properties in contrast with state-of-the-art methods relying on external field variations, moving one step closer to acquiring autonomous shape-shifting capabilities of biological systems.Comment: For SI, see https://drive.google.com/file/d/1MH0kURA_OiOaePDQC06Eua1aG3kkEBV4/view?usp=sharin

Dirac Fermions and Domain Wall Defects in 2+1 Dimensions

We investigate some properties of a system of Dirac fermions in 2+1 dimensions, with a space dependent mass having domain wall like defects.These defects are defined by the loci of the points where the mass changes sign. In general, they will be curves lying on the spatial plane. We show how to treat the dynamics of the fermions in such a way that the existence of localized fermionic zero modes on the defects is transparent. Moreover, effects due to the higher, non zero modes, can be quantitatively studied. We also consider the relevance of the profile of the mass near the region where it changes sign. Finally, we apply our general results to the calculation of the induced fermionic current, in the linear response approximation, in the presence of an external electric field and defects.Comment: 23 pages, 1 Postscript figur

Tunneling between fermionic vacua and the overlap formalism

The probability amplitude for tunneling between the Dirac vacua corresponding to different signs of a parity breaking fermionic mass $M$ in $2+1$ dimensions is studied, making contact with the continuum overlap formulation for chiral determinants. It is shown that the transition probability in the limit when $M \to \infty$ corresponds, via the overlap formalism, to the squared modulus of a chiral determinant in two Euclidean dimensions. The transition probabilities corresponding to two particular examples: fermions on a torus with twisted boundary conditions, and fermions on a disk in the presence of an external constant magnetic field are evaluated.Comment: Reference added. 12 pages, LateX, no figure

Finite temperature regularization

We present a non-perturbative regularization scheme for Quantum Field Theories which amounts to an embedding of the originally unregularized theory into a spacetime with an extra compactified dimensions of length L ~ Lambda^{-1} (with Lambda an ultraviolet cutoff), plus a doubling in the number of fields, which satisfy different periodicity conditions and have opposite Grassmann parity. The resulting regularized action may be interpreted, for the fermionic case, as corresponding to a finite-temperature theory with a supersymmetry, which is broken because of the boundary conditions. We test our proposal in a perturbative calculation (the vacuum polarization graph for a D-dimensional fermionic theory) and in a non-perturbative one (the chiral anomaly).Comment: 17 pages, LaTeX fil

QCD_4 From a Five-Dimensional Point of View

We propose a 5-dimensional definition for the physical 4D-Yang-Mills theory. The fifth dimension corresponds to the Monte-Carlo time of numerical simulations of QCD_4. The 5-dimensional theory is a well-defined topological quantum field theory that can be renormalized at any given finite order of perturbation theory. The relation to non-perturbative physics is obtained by expressing the theory on a lattice, a la Wilson. The new fields that must be introduced in the context of a topological Yang-Mills theory have a simple lattice expression. We present a 5-dimensional critical limit for physical correlation functions and for dynamical auto-correlations, which allows new Monte-Carlo algorithm based on the time-step in lattice units given by \e = g_0^{-13/11} in pure gluodynamics. The gauge-fixing in five dimensions is such that no Gribov ambiguity occurs. The weight is strictly positive, because all ghost fields have parabolic propagators and yield trivial determinants. We indicate how our 5-dimensional description of the Yang-Mills theory may be extended to fermions.Comment: 45 page

Shortgrass Steppe LTER VI: examining ecosystem persistence and responses to global change, 2010-2014 proposal

Includes bibliographical references.The SGS-LTER research site was established in 1980 by researchers at Colorado State University as part of a network of long-term research sites within the US LTER Network, supported by the National Science Foundation. Scientists within the Natural Resource Ecology Lab, Department of Forest and Rangeland Stewardship, Department of Soil and Crop Sciences, and Biology Department at CSU, California State Fullerton, USDA Agricultural Research Service, University of Northern Colorado, and the University of Wyoming, among others, have contributed to our understanding of the structure and functions of the shortgrass steppe and other diverse ecosystems across the network while maintaining a common mission and sharing expertise, data and infrastructure.The Shortgrass Steppe Long-term Ecological Research (SGS-LTER) program focuses on how grassland ecosystems function and persist or change in the face of global change. Our conceptual framework asserts that climate, physiography, grazing, fire and landuse, operating over different spatial and temporal scales, are the dominant determinants of the structure, function, and persistence of the SGS. Using the shortgrass steppe (SGS) ecosystem of the North American Great Plains as a model, we seek to (1) identify the ecological attributes of grasslands that historically have resulted in their persistence and (2) understand these attributes in ways that will allow us to identify area of vulnerability and better forecast the future of grasslands in the face of global change. Given its geographic extent and history, the SGS encapsulates many of the features of a system driven by social-ecological interactions and the vulnerabilities of semiarid grasslands to global change. Our overarching question is: How will structure and function of the SGS respond to expected changes in climate, management, and land-use, and what will be the consequences

Orbital Evolution of Compact White Dwarf Binaries

The new-found prevalence of extremely low mass (ELM, Mhe<0.2 Msun) helium white dwarfs (WDs) in tight binaries with more massive WDs has raised our interest in understanding the nature of their mass transfer. Possessing small (Menv~1e-3 Msun) but thick hydrogen envelopes, these objects have larger radii than cold WDs and so initiate mass transfer of H-rich material at orbital periods of 6-10 minutes. Building on the original work of D'Antona et al., we confirm the 1e6 yr period of continued inspiral with mass transfer of H-rich matter and highlight that the inspiraling direct-impact double WD binary HM Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion under mass loss, thus enabling a larger range of donor masses that can stably transfer matter and become a He mass transferring AM CVn binary. Even once in the long-lived AM CVn mass transferring stage, these He WDs have larger radii due to their higher entropy from the prolonged H burning stage.Comment: 9 pages, 8 figures. Accepted for publication in the Astrophysical Journa

Gut microbiome composition in the Hispanic Community Health Study/Study of Latinos is shaped by geographic relocation, environmental factors, and obesity.

Background: Hispanics living in the USA may have unrecognized potential birthplace and lifestyle influences on the gut microbiome. We report a cross-sectional analysis of 1674 participants from four centers of the Hispanic Community Health Study/Study of Latinos (HCHS/SOL), aged 18 to 74 years old at recruitment.Results: Amplicon sequencing of 16S rRNA gene V4 and fungal ITS1 fragments from self-collected stool samples indicate that the host microbiome is determined by sociodemographic and migration-related variables. Those who relocate from Latin America to the USA at an early age have reductions in Prevotella to Bacteroides ratios that persist across the life course. Shannon index of alpha diversity in fungi and bacteria is low in those who relocate to the USA in early life. In contrast, those who relocate to the USA during adulthood, over 45 years old, have high bacterial and fungal diversity and high Prevotella to Bacteroides ratios, compared to USA-born and childhood arrivals. Low bacterial diversity is associated in turn with obesity. Contrasting with prior studies, our study of&nbsp;the Latino population shows increasing Prevotella to Bacteroides ratio with greater obesity. Taxa within Acidaminococcus, Megasphaera, Ruminococcaceae, Coriobacteriaceae, Clostridiales, Christensenellaceae, YS2 (Cyanobacteria), and Victivallaceae are significantly associated with both obesity and earlier exposure to the USA, while Oscillospira and Anaerotruncus show paradoxical associations with both obesity and late-life introduction to the USA.Conclusions: Our analysis of the gut microbiome of Latinos demonstrates unique features that might be responsible for health disparities affecting Hispanics living in the USA