Physics Commons^™

Isotropization In Brane Gas Cosmology, Scott Watson, Robert H. Brandenberger

Physics - All Scholarship

Brane Gas Cosmology (BGC) is an approach to unifying string theory and cosmology in which matter is described by a gas of strings and branes in a dilaton gravity background. The Universe is assumed to start out with all spatial dimensions compact and small. It has previously been shown that in this context, in the approximation of neglecting inhomogeneities and anisotropies, there is a dynamical mechanism which allows only three spatial dimensions to become large. However, previous studies do not lead to any conclusions concerning the isotropy or anisotropy of these three large spatial dimensions. Here, we generalize the equations …

Stripes From (Noncommutative) Stars, Simon Catterall, J. Ambjorn

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We show that lattice regularization of noncommutative field theories can be used to study non-perturbative vacuum phases. Specifically we provide evidence for the existence of a striped phase in two-dimensional noncommutative scalar field theory

Hysteresis In Driven Disordered Systems: From Plastic Depinning To Magnets, M. Cristina Marchetti, Karin A. Dahmen

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We study the dynamics of a viscoelastic medium driven through quenched disorder by expanding about mean field theory in $6-\epsilon$ dimensions. The model exhibits a critical point separating a region where the dynamics is hysteretic with a macroscopic jump between strongly pinned and weakly pinned states, from a region where the sliding state is unique and no jump occurs. The disappearance of the jump at the critical point is described by universal exponents. As suggested in \onlinecite{MMP00}, the model appears to be in the same universality class as the zero-temperature random field Ising model of hysteresis in magnets.

Precision Measurement Of Energy And Position Resolutions Of The Btev Electromagnetic Calorimeter Prototype, Raymond Mountain, K. Khroustalev, V.A. Batarin, T. Brennan

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The energy dependence of the energy and position resolutions of the electromagnetic calorimeter prototype made of lead tungstate crystals produced in Bogoroditsk (Russia) and Shanghai (China) is presented. These measurementswere carried out at the Protvino accelerator using a 1 to 45 GeV electron beam. The crystals were coupled to photomultiplier tubes. The dependence of energy and position resolutions on different factors as well as the measured electromagnetic shower lateral profile are presented.

Effects Of Light Scalar Mesons In Η\To 3Π Decay, Joseph Schechter, Abdou Abdel-Rehim, Deirdre Black, Amir H. Fariborz

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We study the role of a possible nonet of light scalar mesons in the still interesting \eta \to 3\pi decay process, with the primary motivation of learning more about the scalars themselves. The framework is a conventional non-linear chiral Lagrangian of pseudoscalars and vectors, extended to include the scalars. The parameters involving the scalars were previously obtained to fit the s-wave \pi\pi and \piK scatterings in the region up to about 1 GeV as well as the strong decay \eta' \to \eta \pi\pi. At first, one might expect a large enhancement from diagrams including a light \sigma(560). However there is …

Crystalline Order On A Sphere And The Generalized Thomson Problem, Mark Bowick, Angelo Cacciuto, David R. Nelson, A. Travesset

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We attack generalized Thomson problems with a continuum formalism which exploits a universal long range interaction between defects depending on the Young modulus of the underlying lattice. Our predictions for the ground state energy agree with simulations of long range power law interactions of the form 1/r^{gamma} (0 < gamma < 2) to four significant digits. The regime of grain boundaries is studied in the context of tilted crystalline order and the generality of our approach is illustrated with new results for square tilings on the sphere.

Development Of A Hybrid Photo-Diode And Its Front-End Electronics For The Btev Experiment, Raymond Mountain

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This paper describes the development of a 163-channel Hybrid Photo-Diode (HPD) to be used in the RICH Detector for the BTEV Experiment. This is a joint development project with DEP, Netherlands. It also reports on the development of associated front-end readout electronics based on the va_btev ASIC, undertaken with IDEAS, Norway. Results from bench tests of the first prototypes are presented.

Construction, Pattern Recognition And Performance Of The Cleo Iii Lif-Tea Rich Detector, Raymond Mountain, R. Ayad, Konstantin Vladimirovich Bukin, A. Efimov

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We briefly describe the design, construction and performance of the LiF-Tea RICH detector built to identify charged particles in the CLEO III experiment. Excellent pion/kaon separation is demonstrated.

Scaling, Domains, And States In The Four-Dimensional Random Field Ising Magnet, Alan Middleton

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The four dimensional Gaussian random field Ising magnet is investigated numerically at zero temperature, using samples up to size $64^4$, to test scaling theories and to investigate the nature of domain walls and the thermodynamic limit. As the magnetization exponent $\beta$ is more easily distinguishable from zero in four dimensions than in three dimensions, these results provide a useful test of conventional scaling theories. Results are presented for the critical behavior of the heat capacity, magnetization, and stiffness. The fractal dimensions of the domain walls at criticality are estimated. A notable difference from three dimensions is the structure of the …

Predictions And Observations In Theories With Varying Couplings, Christian Armendariz-Picon

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We consider a toy universe containing conventional matter and an additional real scalar field, and discuss how the requirements of gauge and diffeomorphism invariance essentially single out a particular set of theories which might describe such a world at low energies. In these theories, fermion masses and g-factors, as well as the electromagnetic coupling turn to be scalar field dependent; fermion charges and the gravitational coupling might be assumed to be constant. We then proceed to study the impact of a time variation of the scalar field on measurements of atomic spectra at high redshifts. Light propagation is not affected …

Exact Lattice Supersymmetry: The Two-Dimensional N=2 Wess-Zumino Model, Simon Catterall, Sergey Karamov

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We study the two-dimensional Wess-Zumino model with extended N=2 supersymmetry on the lattice. The lattice prescription we choose has the merit of preserving {\it exactly} a single supersymmetric invariance at finite lattice spacing a. Furthermore, we construct three other transformations of the lattice fields under which the variation of the lattice action vanishes to O(ga^2) where g is a typical interaction coupling. These four transformations correspond to the two Majorana supercharges of the continuum theory. We also derive lattice Ward identities corresponding to these exact and approximate symmetries. We use dynamical fermion simulations to check the equality of the massgaps …

Vector Meson Dominance Model For Radiative Decays Involving Light Scalar Mesons, Joseph Schechter, Deirdre Black, Masayasu Harada

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We study a vector dominance model which predicts a fairly large number of currently interesting decay amplitudes of the types S -> \gamma \gamma, V -> S \gamma and S -> V \gamma, where S and V denote scalar and vector mesons, in terms of three parameters. As an application, the model makes it easy to study in detail a recent proposal to boost the ratio Gamma(phi -> f_0 gamma) / Gamma(phi -> a_0 gamma) by including the isospin violating a_0 - f_0 mixing. However we find that this effect is actually small in our model.

Clash Of Symmetries On The Brane, Aharon Davidson, B. F. Toner, R. R. Volkas, K. C. Wali

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If our 3 + 1-dimensional universe is a brane or domain wall embedded in a higher dimensional space, then a phenomenon we term the “clash of symmetries” provides a new method of breaking some continuous symmetries. A global

Gcts ⊗ Gdiscrete symmetry is spontaneously broken to Hcts ⊗ Hdiscrete, where the continuous subgroup Hcts can be embedded in several different ways in the parent group Gcts, and Hdiscrete < Gdiscrete. A certain class of topological domain wall solutions connect two vacua that are invariant under differently embedded Hcts subgroups. There is then enhanced symmetry breakdown to the intersection of these two subgroups on the domain wall.This is the “clash”. In the brane limit, we obtain a configuration with Hcts symmetries in the bulk but the smaller intersection symmetry on the brane itself. We illustrate this idea using a permutation symmetric three-Higgstriplet toy model exploiting the distinct I−, U− and V − spin U(2) subgroups of U(3). The three disconnected portions of the vacuum manifold can be treated symmetrically through the construction of a three-fold planar domain wall junction configuration, with our universe at the nexus. A possible con-

Determining The Locus For Photocarrier Recombination In Dye-Sensitized Solar Cells, Kai Zhu, Eric A. Schiff, N. G. Park, J. Van De Lagemaat, A. J. Frank

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We present intensity-modulated photocurrent and infrared transmittance measurements on dye-sensitized solar cells based on a mesoporous titania (TiO2) matrix immersed in an iodine-based electrolyte. Under short-circuit conditions, we show that an elementary analysis accurately relates the two measurements. Under open-circuit conditions, infrared transmittance, and photovoltage measurements yield information on the characteristic depth at which electrons recombine with ions (the ‘‘locus of recombination’’). For one particular series of samples recombination occurred near the substrate supporting the titania film, as opposed to homogeneously throughout the film.

Infrared Charge-Modulation Spectroscopy Of Defects In Phosphorus Doped Amorphous Silicon, Kai Zhu, Eric A. Schiff, G. Ganguly

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We present infrared charge-modulation absorption spectra on phosphorus-doped amorphous silicon (a-Si:H:P) with doping levels between 0.17% - 5%. At higher doping levels (1% - 5%) we find a sharp spectral line near 0.75 eV with a width of 0.1 eV. We attribute this line to the internal optical transitions of a complex incorporating four fold coordinated phosphorus and a dangling bond. This line is barely detectable in samples with lower doping levels (below 1%). In these samples a much broader line dominates the spectrum that we attribute to uncomplexed dopants. The relative strength of the two spectral features is in …

Thermionic Emission Model For Interface Effects On The Open-Circuit Voltage Of Amorphous Silicon Based Solar Cells, Eric A. Schiff

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We present computer modeling for effects of the p/i interface upon the open-circuit voltage VOC in amorphous silicon based pin solar cells. We show that the modeling is consistent with measurements on the intensitydependence for the interface effect, and we present an interpretation for the modeling based on thermionic emission of electrons over the electrostatic barrier at the p/i interface. We present additional modeling of the relation of VOC with the intrinsic layer bandgap EG. The experimental correlation for optimized cells is VOC = (EG/e)-0.79. The correlation is simply explained if VOC in these cells is determined by the intrinsic …

Photocarrier Drift Mobility Measurements And Electron Localization In Nanoporous Silicon, P. N. Rao, Eric A. Schiff, L. Tsybeskov, P. Fauchet

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We report photocarrier time-of-flight measurements in diode structures made of highly porous crystalline silicon. The corresponding electron and hole drift mobilities are very small ð<104 cm2=V sÞ compared to homogeneous crystalline silicon. The mobilities are dispersive (i.e., having a power-law decay with time or length-scale), but are only weakly temperature-dependent. The dispersion parameter lies in the range 0.55–0.65 for both electrons and holes. We conclude that the drift mobilities are limited by the nanoporous geometry, and not by disorder-induced localized states acting as traps. This conclusion is surprising in the context of luminescence models based on radiative recombination of localized excitons.