Simulation of a modified Hubbard model with a chemical potential using a meron-cluster algorithm

We show how a variant of the Hubbard model can be simulated using a meron-cluster algorithm. This provides a major improvement in our ability to determine the behavior of these types of models. We also present some results that clearly demonstrate the existence of a superconducting state in this model.Comment: 9 pages, Lattice2002(plenary

Pathogenic Potential to Humans of Bovine Escherichia coli O26, Scotland

Escherichia coli O26 and O157 have similar overall prevalences in cattle in Scotland, but in humans, Shiga toxin–producing E. coli O26 infections are fewer and clinically less severe than E. coli O157 infections. To investigate this discrepancy, we genotyped E. coli O26 isolates from cattle and humans in Scotland and continental Europe. The genetic background of some strains from Scotland was closely related to that of strains causing severe infections in Europe. Nonmetric multidimensional scaling found an association between hemolytic uremic syndrome (HUS) and multilocus sequence type 21 strains and confirmed the role of stx<sub>2</sub> in severe human disease. Although the prevalences of E. coli O26 and O157 on cattle farms in Scotland are equivalent, prevalence of more virulent strains is low, reducing human infection risk. However, new data on E. coli O26–associated HUS in humans highlight the need for surveillance of non-O157 enterohemorrhagic E. coli and for understanding stx<sub>2</sub> phage acquisition

Meron-Cluster Approach to Systems of Strongly Correlated Electrons

Numerical simulations of strongly correlated electron systems suffer from the notorious fermion sign problem which has prevented progress in understanding if systems like the Hubbard model display high-temperature superconductivity. Here we show how the fermion sign problem can be solved completely with meron-cluster methods in a large class of models of strongly correlated electron systems, some of which are in the extended Hubbard model family and show s-wave superconductivity. In these models we also find that on-site repulsion can even coexist with a weak chemical potential without introducing sign problems. We argue that since these models can be simulated efficiently using cluster algorithms they are ideal for studying many of the interesting phenomena in strongly correlated electron systems.Comment: 36 Pages, 13 figures, plain Late

The IUCF Cooler Project

This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

Comparison of two non-primitive methods for path integral simulations: Higher-order corrections vs. an effective propagator approach

Two methods are compared that are used in path integral simulations. Both methods aim to achieve faster convergence to the quantum limit than the so-called primitive algorithm (PA). One method, originally proposed by Takahashi and Imada, is based on a higher-order approximation (HOA) of the quantum mechanical density operator. The other method is based upon an effective propagator (EPr). This propagator is constructed such that it produces correctly one and two-particle imaginary time correlation functions in the limit of small densities even for finite Trotter numbers P. We discuss the conceptual differences between both methods and compare the convergence rate of both approaches. While the HOA method converges faster than the EPr approach, EPr gives surprisingly good estimates of thermal quantities already for P = 1. Despite a significant improvement with respect to PA, neither HOA nor EPr overcomes the need to increase P linearly with inverse temperature. We also derive the proper estimator for radial distribution functions for HOA based path integral simulations.Comment: 17 pages, latex, 6 postscript figure

The IUCF Cooler Project

This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

The Weak Charge of the Proton and New Physics

We address the physics implications of a precision determination of the weak charge of the proton, QWP, from a parity violating elastic electron proton scattering experiment to be performed at the Jefferson Laboratory. We present the Standard Model (SM) expression for QWP including one-loop radiative corrections, and discuss in detail the theoretical uncertainties and missing higher order QCD corrections. Owing to a fortuitous cancellation, the value of QWP is suppressed in the SM, making it a unique place to look for physics beyond the SM. Examples include extra neutral gauge bosons, supersymmetry, and leptoquarks. We argue that a QWP measurement will provide an important complement to both high energy collider experiments and other low energy electroweak measurements. The anticipated experimental precision requires the knowledge of the order alpha_s corrections to the pure electroweak box contributions. We compute these contributions for QWP, as well as for the weak charges of heavy elements as determined from atomic parity violation.Comment: 22 pages of LaTeX, 5 figure

Computer modeling of diabetes and Its transparency: a report on the Eighth Mount Hood Challenge

Objectives The Eighth Mount Hood Challenge (held in St. Gallen, Switzerland, in September 2016) evaluated the transparency of model input documentation from two published health economics studies and developed guidelines for improving transparency in the reporting of input data underlying model-based economic analyses in diabetes. Methods Participating modeling groups were asked to reproduce the results of two published studies using the input data described in those articles. Gaps in input data were filled with assumptions reported by the modeling groups. Goodness of fit between the results reported in the target studies and the groups’ replicated outputs was evaluated using the slope of linear regression line and the coefficient of determination (R2). After a general discussion of the results, a diabetes-specific checklist for the transparency of model input was developed. Results Seven groups participated in the transparency challenge. The reporting of key model input parameters in the two studies, including the baseline characteristics of simulated patients, treatment effect and treatment intensification threshold assumptions, treatment effect evolution, prediction of complications and costs data, was inadequately transparent (and often missing altogether). Not surprisingly, goodness of fit was better for the study that reported its input data with more transparency. To improve the transparency in diabetes modeling, the Diabetes Modeling Input Checklist listing the minimal input data required for reproducibility in most diabetes modeling applications was developed. Conclusions Transparency of diabetes model inputs is important to the reproducibility and credibility of simulation results. In the Eighth Mount Hood Challenge, the Diabetes Modeling Input Checklist was developed with the goal of improving the transparency of input data reporting and reproducibility of diabetes simulation model results

Taxonomy of Penicillium section Citrina

Species of Penicillium section Citrina have a worldwide distribution and occur commonly in soils. The section is here delimited using a combination of phenotypic characters and sequences of the nuclear ribosomal RNA gene operon, including the internal transcribed spacer regions ITS1 and ITS2, the 5.8S nrDNA (ITS) and partial RPB2 sequences. Species assigned to section Citrina share the production of symmetrically biverticillate conidiophores, flask shaped phialides (7.0–9.0 μm long) and relatively small conidia (2.0–3.0 μm diam). Some species can produce greyish-brown coloured cleistothecia containing flanged ascospores. In the present study, more than 250 isolates presumably belonging to section Citrina were examined using a combined analysis of phenotypic and physiological characters, extrolite profiles and ITS, β-tubulin and/or calmodulin sequences. Section Citrina includes 39 species, and 17 of those are described here as new. The most important phenotypic characters for distinguishing species are growth rates and colony reverse colours on the agar media CYA, MEA and YES; shape, size and ornamentation of conidia and the production of sclerotia or cleistothecia. Temperature-growth profiles were made for all examined species and are a valuable character characters for species identification. Species centered around P. citrinum generally have a higher maximum growth temperature (33–36 °C) than species related to P. westlingii (27–33 °C). Extrolite patterns and partial calmodulin and β-tubulin sequences can be used for sequence based identification and resolved all species. In contrast, ITS sequences were less variable and only 55 % of the species could be unambiguously identified with this locus