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Articles 1 - 29 of 29

Full-Text Articles in Physics

Linear Perturbations In Brane Gas Cosmology, Scott Watson, Robert H. Brandenberger Dec 2003

Linear Perturbations In Brane Gas Cosmology, Scott Watson, Robert H. Brandenberger

Physics

We consider the effect of string inhomogeneities on the time dependent background of Brane Gas Cosmology. We derive the equations governing the linear perturbations of the dilaton-gravity background in the presence of string matter sources. We focus on long wavelength fluctuations and find that there are no instabilities. Thus, the predictions of Brane Gas Cosmology are robust against the introduction of linear perturbations. In particular, we find that the stabilization of the extra dimensions (moduli) remains valid in the presence of dilaton and string perturbations.


Domain Wall Solutions With Abelian Gauge Fields, J. S. Rozowsky, R. R. Volkas, K. C. Wali Nov 2003

Domain Wall Solutions With Abelian Gauge Fields, J. S. Rozowsky, R. R. Volkas, K. C. Wali

Physics

We study kink (domain wall) solutions in a model consisting of two complex scalar fields coupled to two independent Abelian gauge fields in a Lagrangian that has $U(1)\times U(1)$ gauge plus $\mathbb{Z}_2$ discrete symmetry. We find consistent solutions such that while the U(1) symmetries of the fields are preserved while in their respective vacua, they are broken on the domain wall. The gauge field solutions show that the domain wall is sandwiched between domains with constant magnetic fields.


Mean Field Theory Of Collective Transport With Phase Slips, Alan Middleton, Karl Saunders, J. M. Schwarz, M. Cristina Marchetti Nov 2003

Mean Field Theory Of Collective Transport With Phase Slips, Alan Middleton, Karl Saunders, J. M. Schwarz, M. Cristina Marchetti

Physics

The driven transport of plastic systems in various disordered backgrounds is studied within mean field theory. Plasticity is modeled using non-convex interparticle potentials that allow for phase slips. This theory most naturally describes sliding charge density waves; other applications include flow of colloidal particles or driven magnetic flux vortices in disordered backgrounds. The phase diagrams exhibit generic phases and phase boundaries, though the shapes of the phase boundaries depend on the shape of the disorder potential. The phases are distinguished by their velocity and coherence: the moving phase generically has finite coherence, while pinned states can be coherent or incoherent ...


Liberating The Inflaton From Primordial Spectrum Constraints, Christian Armendariz-Picon Oct 2003

Liberating The Inflaton From Primordial Spectrum Constraints, Christian Armendariz-Picon

Physics

I discuss a mechanism that renders the spectral index of the primordial spectrum and the inflationary stage independent of each other. If a scalar field acquires an appropriate time-dependent mass, it is possible to generate an adiabatic, Gaussian scale invariant spectrum of density perturbations during any stage of inflation. As an illustration, I present a simple model where the time-dependent mass arises from the coupling of the inflaton to a second scalar. The mechanism I propose might help to implement a successful inflationary scenario in particle physics theories that do not yield slow-roll potentials.


Curvature-Induced Defect Unbinding In Toroidal Geometries, Mark Bowick, David R. Nelson, Alex Travesset Sep 2003

Curvature-Induced Defect Unbinding In Toroidal Geometries, Mark Bowick, David R. Nelson, Alex Travesset

Physics

Toroidal templates such as vesicles with hexatic bond orientational order are discussed. The total energy including disclination charges is explicitly computed for hexatic order embedded in a toroidal geometry. Related results apply for tilt or nematic order on the torus in the one Frank constant approximation. Although there is no topological necessity for defects in the ground state, we find that excess disclination defects are nevertheless energetically favored for fat torii or moderate vesicle sizes. Some experimental consequences are discussed.


Large Nc And Chiral Dynamics, Joseph Schechter, Masayasu Harada, Francesco Sannino Sep 2003

Large Nc And Chiral Dynamics, Joseph Schechter, Masayasu Harada, Francesco Sannino

Physics

We study the dependence on the number of colors of the leading pi pi scattering amplitude in chiral dynamics. We demonstrate the existence of a critical number of colors for and above which the low energy pi pi scattering amplitude computed from the simple sum of the current algebra and vector meson terms is crossing symmetric and unitary at leading order in a truncated and regularized 1/Nc expansion. The critical number of colors turns out to be Nc=6 and is insensitive to the explicit breaking of chiral symmetry. Below this critical value, an additional state is needed to ...


Percolation Of Satisfiability In Finite Dimensions, Alan Middleton, J. M. Schwarz Sep 2003

Percolation Of Satisfiability In Finite Dimensions, Alan Middleton, J. M. Schwarz

Physics

The satisfiability and optimization of finite-dimensional Boolean formulas are studied using percolation theory, rare region arguments, and boundary effects. In contrast with mean-field results, there is no satisfiability transition, though there is a logical connectivity transition. In part of the disconnected phase, rare regions lead to a divergent running time for optimization algorithms. The thermodynamic ground state for the NP-hard two-dimensional maximum-satisfiability problem is typically unique. These results have implications for the computational study of disordered materials.


Exact Lattice Supersymmetry From Topological Field Theory, Simon Catterall Sep 2003

Exact Lattice Supersymmetry From Topological Field Theory, Simon Catterall

Physics

We discuss the connection between supersymmetric field theories and topological field theories and show how this connection may be used to construct local lattice field theories which maintain an exact supersymmetry. It is shown how metric independence of the continuum topological field theory allows us to derive the lattice theory by blocking out of the continuum in a deformed geometry. This, in turn, allows us to prove the cut-off independence of certain supersymmetric Ward identities.


Stabilization Of Extra Dimensions At Tree Level, Scott Watson, Robert H. Brandenberger Sep 2003

Stabilization Of Extra Dimensions At Tree Level, Scott Watson, Robert H. Brandenberger

Physics

By considering the effects of string winding and momentum modes on a time dependent background, we find a method by which six compact dimensions become stabilized naturally at the self-dual radius while three dimensions grow large.


Scale Invariance Without Inflation?, Christian Armendariz-Picon, Eugene A. Lim Sep 2003

Scale Invariance Without Inflation?, Christian Armendariz-Picon, Eugene A. Lim

Physics

We propose a new alternative mechanism to seed a scale invariant spectrum of primordial density perturbations that does not rely on inflation. In our scenario, a perfect fluid dominates the early stages of an expanding, non-inflating universe. Because the speed of sound of the fluid decays, perturbations are left frozen behind the sound horizon, with a spectral index that depends on the fluid equation of state. We explore here a toy model that realizes this idea. Although the model can explain an adiabatic, Gaussian, scale invariant spectrum of primordial perturbations, it turns out that in its simplest form it cannot ...


Anisotropic Inflation And The Origin Of Four Large Dimensions, Christian Armendariz-Picon, Vikram Duvvuri Aug 2003

Anisotropic Inflation And The Origin Of Four Large Dimensions, Christian Armendariz-Picon, Vikram Duvvuri

Physics

In the context of (4+d)-dimensional general relativity, we propose an inflationary scenario wherein 3 spatial dimensions grow large, while d extra dimensions remain small. Our model requires that a self-interacting d-form acquire a vacuum expectation value along the extra dimensions. This causes 3 spatial dimensions to inflate, whilst keeping the size of the extra dimensions nearly constant. We do not require an additional stabilization mechanism for the radion, as stable solutions exist for flat, and for negatively curved compact extra dimensions. From a four-dimensional perspective, the radion does not couple to the inflaton; and, the small amplitude of ...


The Shapes Of Dirichlet Defects, Mark Bowick, Antonio De Felice, Mark Trodden Jun 2003

The Shapes Of Dirichlet Defects, Mark Bowick, Antonio De Felice, Mark Trodden

Physics

If the vacuum manifold of a field theory has the appropriate topological structure, the theory admits topological structures analogous to the D-branes of string theory, in which defects of one dimension terminate on other defects of higher dimension. The shapes of such defects are analyzed numerically, with special attention paid to the intersection regions. Walls (co-dimension 1 branes) terminating on other walls, global strings (co-dimension 2 branes) and local strings (including gauge fields) terminating on walls are all considered. Connections to supersymmetric field theories, string theory and condensed matter systems are pointed out.


Radiative Decays Involving A0(980) And F0(980) In The Vector Meson Dominance Model, Joseph Schechter, Deirdre Black, Masayasu Harada Jun 2003

Radiative Decays Involving A0(980) And F0(980) In The Vector Meson Dominance Model, Joseph Schechter, Deirdre Black, Masayasu Harada

Physics

We summarize some features of the vector meson dominance model which was recently proposed for studying radiative decays involving the scalar mesons. Using the experimental values of \Gamma(a_0 \to \gamma\gamma), \Gamma(f_0 \to \gamma\gamma) and \Gamma(\phi \to a_0 \gamma) as inputs, we show that the model predicts a large hierarchy between \Gamma(a_0 \to \omega \gamma) and \Gamma(a_0 \to \rho \gamma) as well as between \Gamma(f_0 \to \omega \gamma) and \Gamma(f_0 \to \rho \gamma).


The Babar Mini, Duncan Brown May 2003

The Babar Mini, Duncan Brown

Physics

BaBar has recently deployed a new event data format referred to as the Mini. The Mini uses efficient packing and aggressive noise suppression to represent the average reconstructed BaBar event in under 7 KBytes. The Mini packs detector information into simple transient data objects, which are then aggregated into roughly 10 composite persistent objects per event. The Mini currently uses Objectivity persistence, and it is being ported to use Root persistence. The Mini contains enough information to support detailed detector studies, while remaining small and fast enough to be used directly in physics analysis. Mini output is customizable, allowing users ...


Study Of Scalar Mesons And Related Radiative Decays, Joseph Schechter, Deirdre Black, Masayasu Harada May 2003

Study Of Scalar Mesons And Related Radiative Decays, Joseph Schechter, Deirdre Black, Masayasu Harada

Physics

After a brief review of the puzzling light scalar meson sector of QCD, a brief summary will be given of a paper concerning radiative decays involving the light scalars. There, a simple vector meson dominance model is constructed in an initial attempt to relate a large number of the radiative decays involving a putative scalar nonet to each other. As an application it is illustrated why a_0(980)-f_0(980) mixing is not expected to greatly alter the f_0/a_0 production ratio for radiative \phi decays.


Comparing The Higgs Sector Of Electroweak Theory With The Scalar Sector Of Low Energy Qcd, Joseph Schechter, Abdou Abdel-Rehim, Deirdre Black, Amir H. Fariborz, Salah Nasri May 2003

Comparing The Higgs Sector Of Electroweak Theory With The Scalar Sector Of Low Energy Qcd, Joseph Schechter, Abdou Abdel-Rehim, Deirdre Black, Amir H. Fariborz, Salah Nasri

Physics

We first review how the simple K-matrix unitarized linear SU(2) sigma model can explain the experimental data in the scalar pi pi scattering channel of QCD up to about 800 MeV. Since it is just a scaled version of the minimal electroweak Higgs sector, which is often treated with the same unitarization method, we interpret the result as support for this approach in the electroweak model with scaled values of tree level Higgs mass up to at least about 2 TeV. We further note that the relevant QCD effective Lagrangian which fits the data to still higher energies using ...


A Lattice Study Of The Two-Dimensional Wess Zumino Model, Simon Catterall, Sergey Karamov May 2003

A Lattice Study Of The Two-Dimensional Wess Zumino Model, Simon Catterall, Sergey Karamov

Physics

We present results from a numerical simulation of the two-dimensional Euclidean Wess-Zumino model. In the continuum the theory possesses N=1 supersymmetry. The lattice model we employ was analyzed by Golterman and Petcher in \cite{susy} where a perturbative proof was given that the continuum supersymmetric Ward identities are recovered without finite tuning in the limit of vanishing lattice spacing. Our simulations demonstrate the existence of important non-perturbative effects in finite volumes which modify these conclusions. It appears that in certain regions of parameter space the vacuum state can contain solitons corresponding to field configurations which interpolate between different classical ...


The Scalar Sector And The Eta -> 3 Pi Problem, Joseph Schechter, Abdou Abdel-Rehim, Deirdre Black, Amir H. Fariborz Apr 2003

The Scalar Sector And The Eta -> 3 Pi Problem, Joseph Schechter, Abdou Abdel-Rehim, Deirdre Black, Amir H. Fariborz

Physics

First, recent work on light scalar mesons, which is of possible interest in connection with the strong coupling region of QCD is briefly discussed. Then a very short highlighting of a paper concerned with an application to the eta -> 3 pi problem is presented.


Collective Transport In Arrays Of Quantum Dots, Alan Middleton, Ned S. Wingreen Apr 2003

Collective Transport In Arrays Of Quantum Dots, Alan Middleton, Ned S. Wingreen

Physics

(WORDS: QUANTUM DOTS, COLLECTIVE TRANSPORT, PHYSICAL EXAMPLE OF KPZ) Collective charge transport is studied in one- and two-dimensional arrays of small normal-metal dots separated by tunnel barriers. At temperatures well below the charging energy of a dot, disorder leads to a threshold for conduction which grows linearly with the size of the array. For short-ranged interactions, one of the correlation length exponents near threshold is found from a novel argument based on interface growth. The dynamical exponent for the current above threshold is also predicted analytically, and the requirements for its experimental observation are described.


Grain Boundary Scars And Spherical Crystallography, Mark Bowick, A. R. Bausch Mar 2003

Grain Boundary Scars And Spherical Crystallography, Mark Bowick, A. R. Bausch

Physics

We describe experimental investigations of the structure of two-dimensional spherical crystals. The crystals, formed by beads self-assembled on water droplets in oil, serve as model systems for exploring very general theories about the minimum energy configurations of particles with arbitrary repulsive interactions on curved surfaces. Above a critical system size we find that crystals develop distinctive high-angle grain boundaries, or scars, not found in planar crystals. The number of excess defects in a scar is shown to grow linearly with the dimensionless system size. The observed slope is expected to be universal, independent of the microscopic potential.


Vacuum Choices And The Predictions Of Inflation, Christian Armendariz-Picon, Eugene A. Lim Mar 2003

Vacuum Choices And The Predictions Of Inflation, Christian Armendariz-Picon, Eugene A. Lim

Physics

In the presence of a short-distance cutoff, the choice of a vacuum state in an inflating, non-de Sitter universe is unavoidably ambiguous. The ambiguity is related to the time at which initial conditions for the mode functions are specified and to the way the expansion of the universe affects those initial conditions. In this paper we study the imprint of these uncertainties on the predictions of inflation. We parametrize the most general set of possible vacuum initial conditions by two phenomenological variables. We find that the generated power spectrum receives oscillatory corrections whose amplitude is proportional to the Hubble parameter ...


On The Initial Conditions For Brane Inflation, Scott Watson, Robert H. Brandenberger, G. Geshnizjani Mar 2003

On The Initial Conditions For Brane Inflation, Scott Watson, Robert H. Brandenberger, G. Geshnizjani

Physics

String theory gives rise to various mechanisms to generate primordial inflation, of which ``brane inflation'' is one of the most widely considered. In this scenario, inflation takes place while two branes are approaching each other, and the modulus field representing the separation between the branes plays the role of the inflaton field. We study the phase space of initial conditions which can lead to a sufficiently long period of cosmological inflation, and find that taking into account the possibility of nonvanishing initial momentum can significantly change the degree of fine tuning of the required initial conditions.


Driven Depinning Of Strongly Disordered Media And Anisotropic Mean-Field Limits, Alan Middleton, M. Cristina Marchetti, Karl Saunders, J. M. Schwarz Feb 2003

Driven Depinning Of Strongly Disordered Media And Anisotropic Mean-Field Limits, Alan Middleton, M. Cristina Marchetti, Karl Saunders, J. M. Schwarz

Physics

Extended systems driven through strong disorder are modeled generically using coarse-grained degrees of freedom that interact elastically in the directions parallel to the driving force and that slip along at least one of the directions transverse to the motion. A realization of such a model is a collection of elastic channels with transverse viscous couplings. In the infinite range limit this model has a tricritical point separating a region where the depinning is continuous, in the universality class of elastic depinning, from a region where depinning is hysteretic. Many of the collective transport models discussed in the literature are special ...


Lattice Supersymmetry And Topological Field Theory, Simon Catterall Jan 2003

Lattice Supersymmetry And Topological Field Theory, Simon Catterall

Physics

It is known that certain theories with extended supersymmetry can be discretized in such a way as to preserve an exact fermionic symmetry. In the simplest model of this kind, we show that this residual supersymmetric invariance is actually a BRST symmetry associated with gauge fixing an underlying local shift symmetry. Furthermore, the starting lattice action is then seen to be entirely a gauge fixing term. The corresponding continuum theory is known to be a topological field theory. We look, in detail, at one example - supersymmetric quantum mechanics which possesses two such BRST symmetries. In this case, we show that ...


Spinors, Inflation, And Non-Singular Cyclic Cosmologies, Christian Armendariz-Picon, Patrick B. Greene Jan 2003

Spinors, Inflation, And Non-Singular Cyclic Cosmologies, Christian Armendariz-Picon, Patrick B. Greene

Physics

We consider toy cosmological models in which a classical, homogeneous, spinor field provides a dominant or sub-dominant contribution to the energy-momentum tensor of a flat Friedmann-Robertson-Walker universe. We find that, if such a field were to exist, appropriate choices of the spinor self-interaction would generate a rich variety of behaviors, quite different from their widely studied scalar field counterparts. We first discuss solutions that incorporate a stage of cosmic inflation and estimate the primordial spectrum of density perturbations seeded during such a stage. Inflation driven by a spinor field turns out to be unappealing as it leads to a blue ...


Amorphous Silicon Based Solar Cells, Xunming Deng, Eric A. Schiff Jan 2003

Amorphous Silicon Based Solar Cells, Xunming Deng, Eric A. Schiff

Physics

Crystalline semiconductors are very well known, including silicon (the basis of the integrated circuits used in modern electronics), Ge (the material of the first transistor), GaAs and the other III-V compounds (the basis for many light emitters), and CdS (often used as a light sensor). In crystals, the atoms are arranged in near-perfect, regular arrays or lattices. Of course, the lattice must be consistent with the underlying chemical bonding properties of the atoms. For example, a silicon atom forms four covalent bonds to neighboring atoms arranged symmetrically about it. This “tetrahedral” configuration is perfectly maintained in the “diamond” lattice of ...


Low-Mobility Solar Cells: A Device Physics Primer With Application To Amorphous Silicon, Eric A. Schiff Jan 2003

Low-Mobility Solar Cells: A Device Physics Primer With Application To Amorphous Silicon, Eric A. Schiff

Physics

The properties of pin solar cells based on photogeneration of charge carriers into lowmobility materials were calculated for two models. Ideal p- and n-type electrode layers were assumed in both cases. The first, elementary case involves only band mobilities and direct electron–hole recombination. An analytical approximation indicates that the power in thick cells rises as the 1 4 power of the lower band mobility, which reflects the buildup of space-charge under illumination. The approximation agrees well with computer simulation. The second model includes exponential bandtail trapping, which is commonly invoked to account for very low hole drift mobilities in ...


Hole Drift-Mobility Measurements In Contemporary Amorphous Silicon, S. Dinca, Eric A. Schiff, V. Vlahos, C. R. Wronski, Q. Yuan Jan 2003

Hole Drift-Mobility Measurements In Contemporary Amorphous Silicon, S. Dinca, Eric A. Schiff, V. Vlahos, C. R. Wronski, Q. Yuan

Physics

We present hole drift-mobility measurements on hydrogenated amorphous silicon from several laboratories. These temperature-dependent measurements show significant variations of the hole mobility for the differing samples. Under standard conditions (displacement/field ratio of 2×10-9 cm2/V), hole mobilities reach values as large as 0.01 cm2/Vs at room-temperature; these values are improved about tenfold over drift-mobilities of materials made a decade or so ago. The improvement is due partly to narrowing of the exponential bandtail of the valence band, but there is presently little other insight into how deposition procedures affect the hole drift-mobility.


Bandtail Limits To Solar Conversion Efficiencies In Amorphous Silicon Solar Cells, Kai Zhu, Weining Wang, Eric A. Schiff, Jianjun Liang, S. Guha Jan 2003

Bandtail Limits To Solar Conversion Efficiencies In Amorphous Silicon Solar Cells, Kai Zhu, Weining Wang, Eric A. Schiff, Jianjun Liang, S. Guha

Physics

We describe a model for a-Si:H based pin solar cells derived primarily from valence bandtail properties. We show how hole drift-mobility measurements and measurements of the temperature-dependence of the open-circuit voltage VOC can be used to estimate the parameters, and we present VOC(T) measurements. We compared the power density under solar illumination calculated with this model with published results for as-deposited a-Si:H solar cells. The agreement is within 4% for a range of thicknesses, suggesting that the power from as-deposited cells is close to the bandtail limit.