Simulating chiral quarks in the epsilon-regime of QCD

We present simulation results for lattice QCD with chiral fermions in small volumes, where the epsilon-expansion of chiral perturbation theory applies. Our data for the low lying Dirac eigenvalues, as well as mesonic correlation functions, are in agreement with analytical predictions. This allows us to extract values for the leading Low Energy Constants F_{pi} and Sigma.Comment: 4 pages, talk presented by W.B. at Baryons04 (Paris, October 25 - 29, 2004); one Ref. adde

Testing a Topology Conserving Gauge Action in QCD

We study lattice QCD with a gauge action, which suppresses small plaquette values. Thus the MC history is confined to a single topological sector over a significant time, while other observables are decorrelated. This enables the cumulation of statistics with a specific topological charge, which is needed for simulations of QCD in the $\epsilon$-regime. The same action may also be useful for simulations with dynamical quarks. The update is performed with a local HMC algorithm.Comment: 3 pages, 3 figures, poster presented by S. Shcheredin at Lattice2004(theory

Thermal momentum distribution from path integrals with shifted boundary conditions

For a thermal field theory formulated in the grand canonical ensemble, the distribution of the total momentum is an observable characterizing the thermal state. We show that its cumulants are related to thermodynamic potentials. In a relativistic system for instance, the thermal variance of the total momentum is a direct measure of the enthalpy. We relate the generating function of the cumulants to the ratio of (a) a partition function expressed as a Matsubara path integral with shifted boundary conditions in the compact direction, and (b) the ordinary partition function. In this form the generating function is well suited for Monte-Carlo evaluation, and the cumulants can be extracted straightforwardly. We test the method in the SU(3) Yang-Mills theory and obtain the entropy density at three different temperatures.Comment: 4 pages, 1 figure, minor revisions; version accepted in PR

Knockout of proton-neutron pairs from 16^{16}O with electromagnetic probes

After recent improvements to the Pavia model of two-nucleon knockout from $^{16}$O with electromagnetic probes the calculated cross sections are compared to experimental data from such reactions. Comparison with data from a measurement of the $^{16}$O(e,e$'$pn) reaction show much better agreement between experiment and theory than was previously observed. In a comparison with recent data from a measurement of the $^{16}$O($\gamma$,pn) reaction the model over-predicts the measured cross section at low missing momentum.Comment: 6 pages, 5 figure

Lattice Gauge Fixing for Parameter Dependent Covariant Gauges

We propose a non-perturbative procedure to fix generic covariant gauges on the lattice. Varying the gauge parameter, this gauge fixing provides a concrete method to check numerically the gauge dependence of correlators measured on the lattice. The new algorithm turns out to converge with a good efficiency. As a preliminary physical result, we find a sensitive dependence of the gluon propagator on the gauge parameter.Comment: 10 pages (LaTeX2e), 5 eps figure

Exploring Topology Conserving Gauge Actions for Lattice QCD

We explore gauge actions for lattice QCD, which are constructed such that the occurrence of small plaquette values is strongly suppressed. By choosing strong bare gauge couplings we arrive at values for the physical lattice spacings of O(0.1 fm). Such gauge actions tend to confine the Monte Carlo history to a single topological sector. This topological stability facilitates the collection of a large set of configurations in a specific sector, which is profitable for numerical studies in the epsilon-regime. The suppression of small plaquette values is also expected to be favourable for simulations with dynamical quarks. We use a local Hybrid Monte Carlo algorithm to simulate such actions, and we present numerical results for the static potential, the physical scale, the topological stability and the kernel condition number of the overlap Dirac operator. In addition we discuss the question of reflection positivity for a class of such gauge actions.Comment: 28 pages, 8 figure

Dispersion relations for the time-fractional Cattaneo-Maxwell heat equation

In this paper, after a brief review of the general theory of dispersive waves in dissipative media, we present a complete discussion of the dispersion relations for both the ordinary and the time-fractional Cattaneo-Maxwell heat equations. Consequently, we provide a complete characterization of the group and phase velocities for these two cases, together with some non-trivial remarks on the nature of wave dispersion in fractional models.Comment: 18 pages, 7 figure

Nucleon-Nucleon Correlations and Two-Nucleon Currents in Exclusive (e,e′NNe,e'NN) Reactions

The contributions of short-range nucleon-nucleon (NN) correlations, various meson exchange current (MEC) terms and the influence of $\Delta$ isobar excitations (isobaric currents, IC) on exclusive two-nucleon knockout reactions induced by electron scattering are investigated. The nuclear structure functions are evaluated for nuclear matter. Realistic NN interactions derived in the framework of One-Boson-Exchange model are employed to evaluate the effects of correlations and MEC in a consistent way. The correlations correlations are determined by solving the Bethe-Goldstone equation. This yields significant contributions to the structure functions W_L and W_T of the (e,e'pn) and (e,e'pp) reactions. These contributions compete with MEC corrections originating from the $\pi$ and $\rho$ exchange terms of the same interaction. Special attention is paid to the so-called 'super parallel' kinematics at momentum transfers which can be measured e.g. at MAMI in Mainz.Comment: 14 pages, 8 figures include

Final State Interaction in Exclusive (e,e′NN)(e,e'NN) Reactions

Contributions of nucleon-nucleon (NN) correlations, meson exchange currents and the residual final state interactions (FSI) on exclusive two-nucleon knock-out reactions induced by electron scattering are investigated. All contributions are derived from the same realistic meson exchange model for the NN interaction. Effects of correlations and FSI are determined in a consistent way by solving the NN scattering equation, the Bethe-Goldstone equation, for two nucleons in nuclear matter. One finds that the FSI re-scattering terms are non-negligible even if the two nucleons are emitted back to back.Comment: 8 pages, 5 figure

The predictive value of psychological assessment of candidates for gastric bypass: A medical chart review

Background and Objectives: Guidelines for bariatric surgery demand a psychological evaluation of applicants. The aim of this study was to evaluate if the presence of "psychological risk factors" predicts postoperative weight loss after gastric bypass. Methods: Medical records of obese women who underwent bariatric surgery between 2000 and 2004 were reviewed. Psychological assessment consisted of a one-hour semi-structured interview, summarized in a written report. Anthropometric assessment at baseline and 6,12,18 and 24 months after surgery included body weight, height and body mass index. Results: The mean BMI of included patients (N = 92) was 46.2 + 6,3 kg/m2 (range 38.4 - 69.7). Based on the psychological assessment, 27% (N = 25) of the patients were classified as having "psychological risk factors" and 28% (N = 26) were diagnosed with a psychiatric diagnosis, most often major depression. Two years after gastric bypass, 16% of patients with "psychological risk factors" achieved an excellent result (%EWL > 75) versus 39% of those without (p < 0.05). About 1 out of 4 patients was in postoperative psychiatric treatment, but only half of them were identified as having "psychological risk factors" at baseline. Weight loss of patients initiating a psychiatric treatment only after surgery was less than of patients who continued psychiatric treatment already initiated before surgery (55.7 + 14.8 versus 66.5 + 14.2 %EWL). Conclusions: A single semi-structured psychological interview may identify patients who are at risk for diminished postoperative weight loss; however, psychological assessment did not identify those patients who were in need of a psychiatric postoperative treatment