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Articles 183751 - 183780 of 295184

Full-Text Articles in Physical Sciences and Mathematics

Complete Six-Gluon Disk Amplitude In Superstring Theory, Stephan Stieberger, Tomasz R. Taylor Jan 2011

Complete Six-Gluon Disk Amplitude In Superstring Theory, Stephan Stieberger, Tomasz R. Taylor

Tomasz Taylor

We evaluate all next-to-maximal helicity violating (NMHV) six-gluon amplitudes in type I open superstring theory in four dimensions, at the disk level, to all orders in α′. Although the computation utilizes supersymmetric Ward identities, the result holds for all compactifications, even for those that break supersymmetry and is completely model-independent. Together with the maximally helicity violating (MHV) amplitudes presented in the previous work, our results provide the complete six-gluon disk amplitude.

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Amplitude For N-Gluon Superstring Scattering, Stephan Stieberger, Tomasz R. Taylor Jan 2011

Amplitude For N-Gluon Superstring Scattering, Stephan Stieberger, Tomasz R. Taylor

Tomasz Taylor

We consider scattering processes involving N gluonic massless states of open superstrings with certain Regge slope α′. At the semi-classical level, the string world-sheet sweeps a disk and N gluons are created or annihilated at the boundary. We present exact expressions for the corresponding amplitudes, valid to all orders in α′, for the so-called maximally helicity violating configurations, with N=4, 5 and N=6. We also obtain the leading O(α′²) string corrections to the zero-slope N-gluon Yang-Mills amplitudes.

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Branes And Fluxes In D=5 Calabi–Yau Compactifications Of M-Theory, Michał Spaliński, Tomasz R. Taylor Jan 2011

Branes And Fluxes In D=5 Calabi–Yau Compactifications Of M-Theory, Michał Spaliński, Tomasz R. Taylor

Tomasz Taylor

We discuss Poincaré three-brane solutions in D = 5 M-Theory compactifications on Calabi-Yau (CY) threefolds with G-fluxes. We show that the vector moduli freeze at an attractor point. In the case with background flux only, the spacetime geometry contains a zero volume singularity with the three-brane and the CY space shrinking simultaneously to a point. This problem can be avoided by including explicit three-brane sources. We consider two cases in detail: a single brane and, when the transverse dimension is compactified on a circle, a pair of branes with opposite tensions.

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Axion Couplings And Effective Cut-Offs In Superstring Compactifications, Mary K. Gaillard, T. R. Taylor Jan 2011

Axion Couplings And Effective Cut-Offs In Superstring Compactifications, Mary K. Gaillard, T. R. Taylor

Tomasz Taylor

We use the linear supermultiplet formalism of supergravity to study axion couplings and chiral anomalies in the context of field-theoretical Lagrangians describing orbifold compactifications beyond the classical approximation. By matching amplitudes computed in the effective low energy theory with the results of string loop calculations we determine the appropriate counterterm in this effective theory that assures modular invariance to all loop order. We use supersymmetry consistency constraints to identify the correct ultra-violet cutoffs for the effective low energy theory. Our results have a simple interpretation in terms of two-loop unification of gauge coupling constants at the string scale.

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Decay Widths Of Lowest Massive Regge Excitations Of Open Strings, Luis A. Anchordoqui, Haim Goldberg, Tomasz R. Taylor Jan 2011

Decay Widths Of Lowest Massive Regge Excitations Of Open Strings, Luis A. Anchordoqui, Haim Goldberg, Tomasz R. Taylor

Tomasz Taylor

With the advent of the LHC there is widespread interest in the discovery potential for physics beyond the standard model. In TeV-scale open string theory, the new physics can be manifest in the excitation and decay of new resonant structures, corresponding to Regge recurrences of standard model particles. An essential input for the prediction of invariant mass spectra of the decay products (which could serve to identify the resonance as a string excitation) are the partial and total widths of the decay products. We present a parameter-free calculation of these widths for the first Regge recurrence of the SU(3) gluon …

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Calculable E−1/Λ Effects, I. Antoniadis, B. Pioline, T. R. Taylor Jan 2011

Calculable E−1/Λ Effects, I. Antoniadis, B. Pioline, T. R. Taylor

Tomasz Taylor

We identify and evaluate a class of physical amplitudes in four-dimensional N = 4 superstring theory, which receive, in the weak coupling limit, contributions of order e−1/λ, where λ is the type II superstring coupling constant. They correspond to four-derivative Ƒ˜1 interaction terms involving the universal type II dilaton supermultiplet. The exact result, obtained by means of a one-loop computation in the dual heterotic theory compactified on T⁶, is compared with the perturbation theory on the type II side, and the e−1/λ contributions are associated to non-perturbative effects of Euclidean solitons (D-branes) wrapped on K3 × T2. The ten-dimensional decompactification …

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Air Shower Simulation Using Geant4 And Commodity Parallel Computing, L. A. Anchordoqui, G. Cooperman, V. Grinberg, T. P. Mccauley, T. Paul, S. Reucroft, J. D. Swain, G. Alverson Jan 2011

Air Shower Simulation Using Geant4 And Commodity Parallel Computing, L. A. Anchordoqui, G. Cooperman, V. Grinberg, T. P. Mccauley, T. Paul, S. Reucroft, J. D. Swain, G. Alverson

John Swain

We present an evaluation of a simulated cosmic ray shower, based on GEANT4 and TOP-C, which tracks all the particles in the shower. TOP-C (Task Oriented Parallel C) provides a framework for parallel algorithm development which makes tractable the problem of following each particle. This method is compared with a simulation program which employs the Hillas thinning algorithm.

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A Pot Of Gold At The End Of The Cosmic "Raynbow"?, L. A. Anchordoqui, M. T. Dova, T. P. Mccauley, T. Paul, S. Reucroft, J. D. Swain Jan 2011

A Pot Of Gold At The End Of The Cosmic "Raynbow"?, L. A. Anchordoqui, M. T. Dova, T. P. Mccauley, T. Paul, S. Reucroft, J. D. Swain

John Swain

We critically review the common belief that ultrahigh energy cosmic rays are protons or atomic nuclei with masses not exceeding that of iron. We find that heavier nuclei are indeed possible, and discuss possible sources and acceleration mechanisms for such primaries. We also show detailed simulations of extensive air showers produced by "superheavy" nuclei, and discuss prospects for their detection in future experiments.

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Constraints On The Tau Neutrino Mass And Mixing From Precise Measurements Of Tau Decay Rates, J. Swain, L. Taylor Jan 2011

Constraints On The Tau Neutrino Mass And Mixing From Precise Measurements Of Tau Decay Rates, J. Swain, L. Taylor

John Swain

We have derived constraints on the tau neutrino mass and fourth generation mixing from an analysis of the partial widths of tau lepton decays, in particular: τ¯→e¯ν⁻eντ, τ¯→μ¯ν⁻μντ, τ →π¯ντ, and τ →K¯ντ. We present predictions for the tau decay widths, allowing for a non-zero tau neutrino mass, mντ, and for mixing with a neutrino of mass mνL > MZ/2, which is parametrised using a Cabibbo-like mixing angle, θL. By comparison of these theoretical predictions with the experimental measurements, we obtain the following bounds at the 90% confidence level: mντ < 42 MeV and sin² θL < 0.014.

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Asymptotic Infrared Fractal Structure Of The Propagator For A Charged Fermion, S. Gulzari, J. Swain, A. Widom Jan 2011

Asymptotic Infrared Fractal Structure Of The Propagator For A Charged Fermion, S. Gulzari, J. Swain, A. Widom

John Swain

It is well known that the long-range nature of the Coulomb interaction makes the definition of asymptotic "in" and "out" states of charged particles problematic in quantum field theory. In particular, the notion of a simple particle pole in the vacuum charged particle propagator is untenable and should be replaced by a more complicated branch cut structure describing an electron interacting with a possibly infinite number of soft photons. Previous work suggests a Dirac propagator raised to a fractional power dependent upon the fine structure constant, however the exponent has not been calculated in a unique gauge invariant manner. It …

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Anomalous Charged Current Couplings Of The Tau And Implications For Tau Compositeness And Two-Higgs Doublet Models, Maria-Teresa Dova, John Swain, Lucas Taylor Jan 2011

Anomalous Charged Current Couplings Of The Tau And Implications For Tau Compositeness And Two-Higgs Doublet Models, Maria-Teresa Dova, John Swain, Lucas Taylor

John Swain

The leptonic branching fractions of the tau lepton are sensitive to anomalous charged current interactions. We use recent experimental measurements to determine the weak charged current magnetic and electric dipole moments and the Michel parameter with unprecedented precision. These results are then used to constrain the tau compositeness scale and the allowed parameter space for Higgs doublet models.

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Constraints On Anomalous Charged Current Couplings, Tau Neutrino Mass And Fourth Generation Mixing From Tau Leptonic Branching Fractions, Maria Teresa Dova, John Swain, Lucas Taylor Jan 2011

Constraints On Anomalous Charged Current Couplings, Tau Neutrino Mass And Fourth Generation Mixing From Tau Leptonic Branching Fractions, Maria Teresa Dova, John Swain, Lucas Taylor

John Swain

We use recent experimental measurements of tau branching fractions to determine the weak charged current magnetic and electric dipole moments of the tau and the Michel parameter η with unprecedented precision. These results are then used to constrain the tau compositeness scale and the allowed parameter space for Higgs doublet models. We also present new constraints on the mass of the tau neutrino and its mixing with a fourth generation neutrino.

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A Depression Before A Bump In The Highest Energy Cosmic Ray Spectrum, L. A. Anchordoqui, M. T. Dova, L. N. Epele, J. D. Swain Jan 2011

A Depression Before A Bump In The Highest Energy Cosmic Ray Spectrum, L. A. Anchordoqui, M. T. Dova, L. N. Epele, J. D. Swain

John Swain

We re-examine the interaction of ultra high energy nuclei with the microwave background radiation. We find that the giant dipole resonance leaves a new signature in the differential energy spectrum of iron sources located around 3 Mpc: A depression before the bump which is followed by the expected cutoff.

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A New Way To Detect The Higgs, S. Reucroft, Y. Srivastava, J. Swain, A. Widom Jan 2011

A New Way To Detect The Higgs, S. Reucroft, Y. Srivastava, J. Swain, A. Widom

John Swain

We describe a new technique to look for evidence of the Higgs mechanism. The usual method involves seeking evidence for the Higgs boson either directly or via the indirect effect that a virtual Higgs boson would have on a variety of Standard Model parameters. The new technique looks for Higgs field effects that are predicted to reduce the masses of heavy particles when they are in the presence of other heavy particles.

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Anisotropies In Ultrahigh Energy Cosmic Rays, John Swain Jan 2011

Anisotropies In Ultrahigh Energy Cosmic Rays, John Swain

John Swain

The present status of anisotropy studies for the highest energy cosmic rays is presented including the first full sky survey. Directions and prospects for the future are also discussed in light of new statistical methods and the last quantities of data expected in the near future from the Pierre Auger Observatory.

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A Method To Determine The Tau Neutrino Helicity Using Polarized Taus, M.-T. Dova, L. N. Epele, H. Fanchiotti, C.A. García Canal, P. E. Lacentre, J. D. Swain Jan 2011

A Method To Determine The Tau Neutrino Helicity Using Polarized Taus, M.-T. Dova, L. N. Epele, H. Fanchiotti, C.A. García Canal, P. E. Lacentre, J. D. Swain

John Swain

A method is presented to extract the tau neutrino helicity, or equivalently, the chirality parameter γVA, independent of any tau polarization which may be present. The method is thus well-suited to measurements using taus produced from the Z⁰ and is complementary to analyses using tau correlations since it provides the sign of the chirality parameter which is otherwise unavailable without recourse to lower energy experiments where taus are unpolarized. Results of Monte Carlo studies and comments regarding the use of technique in experiments are also included.

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A Mechanism For Lubrication Between Surfaces With Atomic Level Roughness, J. B. Sokoloff Jan 2011

A Mechanism For Lubrication Between Surfaces With Atomic Level Roughness, J. B. Sokoloff

Jeffrey Sokoloff

It is proposed that lubricant molecules adsorbed on an interface between two asperities in contact, which is rough on the atomic scale, can switch the interface from the strong to weak pinning regime, resulting in a large reduction in the static friction. This is proposed as a possible mechanism for boundary lubrication.

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Dark Matter As A Guide Toward A Light Gluino At The Lhc, Daniel Feldman, Gordon Kane, Ran Lu, Brent T. Nelson Jan 2011

Dark Matter As A Guide Toward A Light Gluino At The Lhc, Daniel Feldman, Gordon Kane, Ran Lu, Brent T. Nelson

Brent Nelson

Motivated by specific connections to dark matter signatures, we study the prospects of observing the presence of a relatively light gluino whose mass is in the range ~ (500 − 900) GeV with a wino-like lightest supersymmetric particle with mass in the range of ~ (170 − 210) GeV. The light gaugino spectra studied here is generally different from other models, and in particular those with a wino dominated LSP, in that here the gluinos can be significantly lighter. The positron excess reported by the PAMELA satellite data is accounted for by annihilations of the wino LSP and their relic …

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Dark Matter Prospects In Deflected Mirage Mediation, Michael Holmes, Brent T. Nelson Jan 2011

Dark Matter Prospects In Deflected Mirage Mediation, Michael Holmes, Brent T. Nelson

Brent Nelson

The recently introduced detected mirage mediation (DMM) model is a string-motivated paradigm in which all three of the major supersymmetry-breaking transmission mechanisms are operative. We begin a systematic exploration of the parameter space of this rich model context, paying special attention to the pattern of gaugino masses which arise. In this work we focus on the dark matter phenomenology of the DMM model as such signals are the least influenced by the model-dependent scalar masses. We find that a large portion of the parameter space in which the three mediation mechanisms have a similar effective mass scale of 1 TeV …

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Coherent Backscattering Near The Two-Dimensional Metal-Insulator Transition, Maryam Rahimi, S. Anissimova, M. R. Sakr, S. V. Kravchenko, T. M. Klapwijk Jan 2011

Coherent Backscattering Near The Two-Dimensional Metal-Insulator Transition, Maryam Rahimi, S. Anissimova, M. R. Sakr, S. V. Kravchenko, T. M. Klapwijk

Sergey Kravchenko

We have studied corrections to conductivity due to the coherent backscattering in low-disordered two-dimensional electron systems in silicon for a range of electron densities including the vicinity of the metal-insulator transition, where the dramatic increase of the spin susceptibility has been observed earlier. We show that the corrections, which exist deeper in the metallic phase, weaken upon approaching to the transition and practically vanish at the critical density, thus suggesting that the localization is suppressed near and at the transition even in zero field.

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Compressibility Of A Two-Dimensional Hole Gas In A Tilted Magnetic Field, Maryam Rahimi, M. Sakr, S. Kravchenko, S. Dultz, H. Jiang Jan 2011

Compressibility Of A Two-Dimensional Hole Gas In A Tilted Magnetic Field, Maryam Rahimi, M. Sakr, S. Kravchenko, S. Dultz, H. Jiang

Sergey Kravchenko

We have measured compressibility of a two-dimensional hole gas in p-GaAs/AlGaAs heterostructure, grown on a (100) surface, in the presence of a tilted magnetic field. It turns out that the parallel component of magnetic field affects neither the spin splitting nor the density of states. We conclude that: (a) g-factor in the parallel magnetic field is nearly zero in this system; and (b) the level of the disorder potential is not sensitive to the parallel component of the magnetic field.

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Conductivity Of A Spin-Polarized Two-Dimensional Electron Liquid In The Ballistic Regime, A. A. Shashkin, E. V. Deviatov, V. T. Dolgopolov, A. A. Kapustin, S. Anissimova, A. Venkatesan, S. V. Kravchenko, T. M. Klapwijk Jan 2011

Conductivity Of A Spin-Polarized Two-Dimensional Electron Liquid In The Ballistic Regime, A. A. Shashkin, E. V. Deviatov, V. T. Dolgopolov, A. A. Kapustin, S. Anissimova, A. Venkatesan, S. V. Kravchenko, T. M. Klapwijk

Sergey Kravchenko

In the ballistic regime, the metallic temperature dependence of the conductivity in a twodimensional electron system in silicon is found to change non-monotonically with the degree of spin polarization. In particular, it fades away just before the onset of complete spin polarization but reappears again in the fully spin-polarized state, being, however, suppressed relative to the zero-field case. Analysis of the degree of the suppression allows one to distinguish between the screening and the interaction-based theories.

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A Metal-Insulator Transition In 2d: Established Facts And Open Questions, S. V. Kravchenko, M. P. Sarachik Jan 2011

A Metal-Insulator Transition In 2d: Established Facts And Open Questions, S. V. Kravchenko, M. P. Sarachik

Sergey Kravchenko

The discovery of a metallic state and a metal-insulator transition (MIT) in two-dimensional (2D) electron systems challenges one of the most influential paradigms of modern mesoscopic physics, namely, that "there is no true metallic behavior in two dimensions". However, this conclusion was drawn for systems of noninteracting or weakly interacting carriers, while in all 2D systems exhibiting the metal-insulator transition, the interaction energy greatly exceeds all other energy scales. We review the main experimental findings and show that, although significant progress has been achieved in our understanding of the MIT in 2D, many open questions remain.

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Transport In Strongly Correlated Two Dimensional Electron Fluids, S. Kravchenko, S. Kivelson, X. Gao, X. Gao Jan 2011

Transport In Strongly Correlated Two Dimensional Electron Fluids, S. Kravchenko, S. Kivelson, X. Gao, X. Gao

Sergey Kravchenko

We present an overview of the measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities, and of some of the theories that have been proposed to account for them. Many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium. Rather, they reflect new physics associated with strong correlation effects, which warrant further study.

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"Forbidden" Transitions Between Quantum Hall And Insulating Phases In P-Sige Heterostructures, M. R. Sakr, Maryam Rahimi, S. V. Kravchenko, P. T. Coleridge, R. L. Williams, J. Lapointe Jan 2011

"Forbidden" Transitions Between Quantum Hall And Insulating Phases In P-Sige Heterostructures, M. R. Sakr, Maryam Rahimi, S. V. Kravchenko, P. T. Coleridge, R. L. Williams, J. Lapointe

Sergey Kravchenko

We show that in dilute metallic p-SiGe heterostructures, magnetic field can cause multiple quantum Hall-insulator-quantum Hall transitions. The insulating states are observed between quantum Hall states with filling factors = 1 and 2 and, for the first time, between = 2 and 3 and between = 4 and 6. The latter are in contradiction with the original global phase diagram for the quantum Hall effect. We suggest that the application of a (perpendicular) magnetic field induces insulating behaviour in metallic p-SiGe heterostructures in the same way as in Si MOSFETs. This insulator is then in competition with, and interrupted by, …

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Comment On "Theory Of Metal-Insulator Transitions In Gated Semiconductors", S. V. Kravchenko, M. P. Sarachik, D. Simonian Jan 2011

Comment On "Theory Of Metal-Insulator Transitions In Gated Semiconductors", S. V. Kravchenko, M. P. Sarachik, D. Simonian

Sergey Kravchenko

No abstract provided.

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Coupled Dynamics Of Voltage And Calcium In Paced Cardiac Cells, Yohannes Shiferaw, Daisuke Sato, Alain Karma Jan 2011

Coupled Dynamics Of Voltage And Calcium In Paced Cardiac Cells, Yohannes Shiferaw, Daisuke Sato, Alain Karma

Alain Karma

We investigate numerically and analytically the coupled dynamics of transmembrane voltage and intracellular calcium cycling in paced cardiac cells using a detailed physiological model and its reduction to a three-dimensional discrete map. The results provide a theoretical framework to interpret various experimentally observed modes of instability ranging from electromechanically concordant and discordant alternans to quasiperiodic oscillations of voltage and calcium.

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Amplitude Equations For Polycrystalline Materials With Interaction Between Composition And Stress, Alain Karma Jan 2011

Amplitude Equations For Polycrystalline Materials With Interaction Between Composition And Stress, Alain Karma

Alain Karma

We investigate the ability of frame-invariant amplitude equations [G. H. Gunaratne, Q. Ouyang, and H. Swinney, Phys. Rev. E 50, 2802 (1994)] to describe quantitatively the evolution of polycrystalline microstructures and we extend this approach to include the interaction between composition and stress. Validations for elemental materials include studies of the Asaro-Tiller-Grinfeld morphological instability of a stressed crystal surface, polycrystalline growth from the melt, grain boundary energies over a wide range of misorientation, and grain boundary motion coupled to shear deformation. Amplitude equations with accelerated strain relaxation in the solid are shown to model accurately the Asaro-Tiller-Grinfeld instability. Polycrystalline growth …

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Black Hole Chromosphere At The Cern Lhc, Luis Anchordoqui, Haim Goldberg Jan 2011

Black Hole Chromosphere At The Cern Lhc, Luis Anchordoqui, Haim Goldberg

Haim Goldberg

If the scale of quantum gravity is near a TeV, black holes will be copiously produced at the LHC. In this work we study the main properties of the light descendants of these black holes. We show that the emitted partons are closely spaced outside the horizon, and hence they do not fragment into hadrons in vacuum but more likely into a kind of quark-gluon plasma. Consequently, the thermal emission occurs far from the horizon, at a temperature characteristic of the QCD scale. We analyze the energy spectrum of the particles emerging from the "chromosphere", and find that the hard …

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Constraints On Unparticle Physics From Solar And Kamland Neutrinos, Luis Anchordoqui, Haim Goldberg Jan 2011

Constraints On Unparticle Physics From Solar And Kamland Neutrinos, Luis Anchordoqui, Haim Goldberg

Haim Goldberg

Interest has been directed recently towards low energy implications of a non-trivial conformal sector of an effective field theory with an IR fixed point (ΛU), manifest in terms of "unparticles" with bizarre properties. We re-examine the implications of the limits on decay lifetimes of solar neutrinos for unparticle interactions. We study in detail the fundamental parameter space (ΛU,M) and derive bounds on the energy scale M characterizing the new physics. We work strictly within the framework where conformal invariance holds down to low energies. We first assume that couplings of the unparticle sector to the Higgs field are suppressed and …

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