Phase structure and monopoles in U(1) gauge theory

We investigate the phase structure of pure compact U(1) lattice gauge theory in 4 dimensions with the Wilson action supplemented by a monopole term. To overcome the suppression of transitions between the phases in the simulations we make the monopole coupling a dynamical variable. We determine the phase diagram and find that the strength of the first order transition decreases with increasing weight of the monopole term, the transition thus ultimately getting of second order. After outlining the appropriate topological characterization of networks of currents lines, we present an analysis of the occurring monopole currents which shows that the phases are related to topological properties.Comment: 22 pages (latex), 14 figures (available upon request), BU-HEP 94-

Quasiperiodic spin-orbit motion and spin tunes in storage rings

We present an in-depth analysis of the concept of spin precession frequency for integrable orbital motion in storage rings. Spin motion on the periodic closed orbit of a storage ring can be analyzed in terms of the Floquet theorem for equations of motion with periodic parameters and a spin precession frequency emerges in a Floquet exponent as an additional frequency of the system. To define a spin precession frequency on nonperiodic synchro-betatron orbits we exploit the important concept of quasiperiodicity. This allows a generalization of the Floquet theorem so that a spin precession frequency can be defined in this case too. This frequency appears in a Floquet-like exponent as an additional frequency in the system in analogy with the case of motion on the closed orbit. These circumstances lead naturally to the definition of the uniform precession rate and a definition of spin tune. A spin tune is a uniform precession rate obtained when certain conditions are fulfilled. Having defined spin tune we define spin-orbit resonance on synchro--betatron orbits and examine its consequences. We give conditions for the existence of uniform precession rates and spin tunes (e.g. where small divisors are controlled by applying a Diophantine condition) and illustrate the various aspects of our description with several examples. The formalism also suggests the use of spectral analysis to ``measure'' spin tune during computer simulations of spin motion on synchro-betatron orbits.Comment: 62 pages, 1 figure. A slight extension of the published versio

Strength of Higher-Order Spin-Orbit Resonances

When polarized particles are accelerated in a synchrotron, the spin precession can be periodically driven by Fourier components of the electromagnetic fields through which the particles travel. This leads to resonant perturbations when the spin-precession frequency is close to a linear combination of the orbital frequencies. When such resonance conditions are crossed, partial depolarization or spin flip can occur. The amount of polarization that survives after resonance crossing is a function of the resonance strength and the crossing speed. This function is commonly called the Froissart-Stora formula. It is very useful for predicting the amount of polarization after an acceleration cycle of a synchrotron or for computing the required speed of the acceleration cycle to maintain a required amount of polarization. However, the resonance strength could in general only be computed for first-order resonances and for synchrotron sidebands. When Siberian Snakes adjust the spin tune to be 1/2, as is required for high energy accelerators, first-order resonances do not appear and higher-order resonances become dominant. Here we will introduce the strength of a higher-order spin-orbit resonance, and also present an efficient method of computing it. Several tracking examples will show that the so computed resonance strength can indeed be used in the Froissart-Stora formula. HERA-p is used for these examples which demonstrate that our results are very relevant for existing accelerators.Comment: 10 pages, 6 figure

Mom-I don't want to hear it: Brain response to maternal praise and criticism in adolescents with major depressive disorder

Recent research has implicated altered neural response to interpersonal feedback as an important factor in adolescent depression, with existing studies focusing on responses to feedback from virtual peers. We investigated whether depressed adolescents differed from healthy youth in neural response to social evaluative feedback from mothers. During neuroimaging, twenty adolescents in a current episode of major depressive disorder (MDD) and 28 healthy controls listened to previously recorded audio clips of their own mothers' praise, criticism and neutral comments. Whole-brain voxelwise analyses revealed that MDD youth, unlike controls, exhibited increased neural response to critical relative to neutral clips in the parahippocampal gyrus, an area involved in episodic memory encoding and retrieval. Depressed adolescents also showed a blunted response to maternal praise clips relative to neutral clips in the parahippocampal gyrus, as well as areas involved in reward and self-referential processing (i.e. Ventromedial prefrontal cortex, precuneus, and thalamus/caudate). Findings suggest that maternal criticism may be more strongly encoded or more strongly activated during memory retrieval related to previous autobiographical instances of negative feedback from mothers in depressed youth compared to healthy youth. Furthermore, depressed adolescents may fail to process the reward value and self-relevance of maternal praise

High-statistics finite size scaling analysis of U(1) lattice gauge theory with Wilson action

We describe the results of a systematic high-statistics Monte-Carlo study of finite-size effects at the phase transition of compact U(1) lattice gauge theory with Wilson action on a hypercubic lattice with periodic boundary conditions. We find unambiguously that the critical exponent nu is lattice-size dependent for volumes ranging from 4^4 to 12^4. Asymptotic scaling formulas yield values decreasing from nu(L >= 4) = 0.33 to nu(L >= 9) = 0.29. Our statistics are sufficient to allow the study of different phenomenological scenarios for the corrections to asymptotic scaling. We find evidence that corrections to a first-order transition with nu=0.25 provide the most accurate description of the data. However the corrections do not follow always the expected first-order pattern of a series expansion in the inverse lattice volume V^{-1}. Reaching the asymptotic regime will require lattice sizes greater than L=12. Our conclusions are supported by the study of many cumulants which all yield consistent results after proper interpretation.Comment: revtex, 12 pages, 9 figure

Chiral transition and monopole percolation in lattice scalar QED with quenched fermions

We study the interplay between topological observables and chiral and Higgs transitions in lattice scalar QED with quenched fermions. Emphasis is put on the chiral transition line and magnetic monopole percolation at strong gauge coupling. We confirm that at infinite gauge coupling the chiral transition is described by mean field exponents. We find a rich and complicated behaviour at the endpoint of the Higgs transition line which hampers a satisfactory analysis of the chiral transition. We study in detail an intermediate coupling, where the data are consistent both with a trivial chiral transition clearly separated from monopole percolation and with a chiral transition coincident with monopole percolation, and characterized by the same critical exponent $\nu \simeq 0.65$. We discuss the relevance (or lack thereof) of these quenched results to our understanding of the \chupiv\ model. We comment on the interplay of magnetic monopoles and fermion dynamics in more general contexts.Comment: 29 pages, 13 figures included, LaTeX2e (elsart

Regular Spectra and Universal Directionality of Emitted Radiation from a Quadrupolar Deformed Microcavity

We have investigated quasi-eigenmodes of a quadrupolar deformed microcavity by extensive numerical calculations. The spectral structure is found to be quite regular, which can be explained on the basis of the fact that the microcavity is an open system. The far-field emission directions of the modes show unexpected similarity irrespective of their distinct shapes in phase space. This universal directionality is ascribed to the influence from the geometry of the unstable manifolds in the corresponding ray dynamics.Comment: 10 pages 11 figure

Phase-transitions induced by easy-plane anisotropy in the classical Heisenberg antiferromagnet on a triangular lattice: a Monte Carlo simulation

We present the results of Monte Carlo simulations for the antiferromagnetic classical XXZ model with easy-plane exchange anisotropy on the triangular lattice, which causes frustration of the spin alignment. The behaviour of this system is similar to that of the antiferromagnetic XY model on the same lattice, showing the signature of a Berezinskii-Kosterlitz-Thouless transition, associated to vortex-antivortex unbinding, and of an Ising-like one due to the chirality, the latter occurring at a slightly higher temperature. Data for internal energy, specific heat, magnetic susceptibility, correlation length, and some properties associated with the chirality are reported in a broad temperature range, for lattice sizes ranging from 24x24 to 120x120; four values of the easy-plane anisotropy are considered. Moving from the strongest towards the weakest anisotropy (1%) the thermodynamic quantities tend to the isotropic model behaviour, and the two transition temperatures decrease by about 25% and 22%, respectively.Comment: 11 pages, 13 figures (embedded by psfig), 3 table

Exclusive diffractive processes and the quark substructure of mesons

Exclusive diffractive processes on the nucleon are investigated within a model in which the quark-nucleon interaction is mediated by Pomeron exchange and the quark substructure of mesons is described within a framework based on the Dyson-Schwinger equations of QCD. The model quark-nucleon interaction has four parameters which are completely determined by high-energy $\pi N$ and $K N$ elastic scattering data. The model is then used to predict vector-meson electroproduction observables. The obtained $\rho$- and $\phi$-meson electroproduction cross sections are in excellent agreement with experimental data. The predicted $q^2$ dependence of $J/\psi$-meson electroproduction also agrees with experimental data. It is shown that confined-quark dynamics play a central role in determining the behavior of the diffractive, vector-meson electroproduction cross section. In particular, the onset of the asymptotic $1/q^4$ behavior of the cross section is determined by a momentum scale that is set by the current-quark masses of the quark and antiquark inside the vector meson. This is the origin of the striking differences between the $q^2$ dependence of $\rho$-, $\phi$- and $J/\psi$-meson electroproduction cross sections observed in recent experiments.Comment: 53 pages, 23 figures, revtex and epsfig. Minor additions to tex

Monte Carlo simulation of a two-dimensional continuum Coulomb gas

We study the classical two-dimensional Coulomb gas model for thermal vortex fluctuations in thin superconducting/superfluid films by Monte Carlo simulation of a grand canonical vortex ensemble defined on a continuum. The Kosterlitz-Thouless transition is well understood at low vortex density, but at high vortex density the nature of the phase diagram and of the vortex phase transition is less clear. From our Monte Carlo data we construct phase diagrams for the 2D Coulomb gas without any restrictions on the vortex density. For negative vortex chemical potential (positive vortex core energy) we always find a Kosterlitz-Thouless transition. Only if the Coulomb interaction is supplemented with a short-distance repulsion, a first order transition line is found, above some positive value of the vortex chemical potential.Comment: 10 pages RevTeX, 7 postscript figures included using eps