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University of Massachusetts Amherst

1998

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Articles 91 - 107 of 107

Full-Text Articles in Physical Sciences and Mathematics

Simulating Gas At High Redshift, N Katz, L Hernquist, Dh Weinberg Jan 1998

Simulating Gas At High Redshift, N Katz, L Hernquist, Dh Weinberg

Astronomy Department Faculty Publication Series

We discuss simulations of gas at high redshift. We briefly review the methods used and the results for quasar absorption lines. We present gas mass functions and galaxy correlation functions for 5 different cosmological models. Galaxies should be detectable at redshifts greater than 2 by SKAI, and measurements of the gas maWe discuss simulations of gas at high redshift. We briefly review the methods used and the results for quasar absorption lines. We present gas mass functions and galaxy correlation functions for 5 different cosmological models. Galaxies should be detectable at redshifts greater than 2 by SKAI, and measurements of …

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Macro-Actions In Reinforcement Learning: An Empirical Analysis, Amy Mcgovern, Richard S. Sutton Jan 1998

Macro-Actions In Reinforcement Learning: An Empirical Analysis, Amy Mcgovern, Richard S. Sutton

Computer Science Department Faculty Publication Series

Several researchers have proposed reinforcement learning methods that obtain advantages in learning by using temporally extended actions, or macro-actions, but none has carefully analyzed what these advantages are. In this paper, we separate and analyze two advantages of using macro-actions in reinforcement learning: the effect on exploratory behavior, independent of learning, and the effect on the speed with which the learning process propagates accurate value information. We empirically measure the separate contributions of these two effects in gridworld and simulated robotic environments. In these environments, both effects were significant, but the effect of value propagation was larger. We also compare …

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Exploiting Relational Structure To Understand Publication Patterns In High-Energy, Amy Mcgovern, Lisa Friedland, Michael Hay, Brian Gallagher, Andrew Fast, Jennifer Neville, David Jensen Jan 1998

Exploiting Relational Structure To Understand Publication Patterns In High-Energy, Amy Mcgovern, Lisa Friedland, Michael Hay, Brian Gallagher, Andrew Fast, Jennifer Neville, David Jensen

Computer Science Department Faculty Publication Series

We analyze publication patterns in theoretical high-energy physics using a relational learning approach. We focus on four related areas: understanding and identifying patterns of citations, examining publication patterns at the author level, predicting whether a paper will be accepted by specific journals, and identifying research communities from the citation patterns and paper text. Each of these analyses contributes to an overall understanding of theoretical highenergy physics.

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Real-Time Reliable Multicast Using Proactive Forward Error Correction, Dan Rubenstein Jan 1998

Real-Time Reliable Multicast Using Proactive Forward Error Correction, Dan Rubenstein

Computer Science Department Faculty Publication Series

Real-Time reliable multicast over a best-effort service network remains a challenging research problem. Most protocols for reliable multicast use repair techniques that result in significant and variable delay, which can lead to missed deadlines in real-time scenarios. In this paper we present a repair technique that combines forward error correction (FEC) with automatic repeat request (ARQ). The novel aspect of the technique is its ability to reduce delay in reliable multicast delivery by sending repairs proactively (i.e., before they are required). The technique requires minimal state at senders and receivers, and no additional active router functionality beyond what is required …

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Heavy Baryon Chiral Perturbation Theory With Light Deltas, Tr Hemmert, Br Holstein, J Kambor Jan 1998

Heavy Baryon Chiral Perturbation Theory With Light Deltas, Tr Hemmert, Br Holstein, J Kambor

Physics Department Faculty Publication Series

We demonstrate how heavy mass methods, previously applied to chiral perturbation theory calculations involving the interactions of nucleons and pions, can be generalized to include interactions with the (1232) in a systematic formalism which we call the `small-scale expansion'.

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Compton Scattering And The Spin Structure Of The Nucleon At Low Energies, Tr Hemmert, Br Holstein, J Kambor, G Knochlein Jan 1998

Compton Scattering And The Spin Structure Of The Nucleon At Low Energies, Tr Hemmert, Br Holstein, J Kambor, G Knochlein

Physics Department Faculty Publication Series

We analyze polarized Compton scattering which provides information on the spin-structure of the nucleon. For scattering processes with photon energies up to 100 MeV the spin-structure dependence can be encoded into four independent parameters—the so-called spin-polarizabilities γi, i=1,…,4, of the nucleon, which we calculate within the framework of the “small scale expansion” in SU(2) baryon chiral perturbation theory. Specific application is made to “forward” and “backward” spin-polarizabilities.

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Search For Physics Beyond The Standard Model Via A Polarization-Asymmetry Correlation Experiment On In-107, B Vereecke, J Camps, N Severijns, J Deutsch, T Otto, J Govaerts, Ba Brown, B Holstein, R Kirchner, O Naviliat-Cuncic, R Prieels, Pa Quin, E Thomas, P De Moor, P Schuurmans, A Van Geert, L Vanneste Jan 1998

Search For Physics Beyond The Standard Model Via A Polarization-Asymmetry Correlation Experiment On In-107, B Vereecke, J Camps, N Severijns, J Deutsch, T Otto, J Govaerts, Ba Brown, B Holstein, R Kirchner, O Naviliat-Cuncic, R Prieels, Pa Quin, E Thomas, P De Moor, P Schuurmans, A Van Geert, L Vanneste

Physics Department Faculty Publication Series

We report on a new precision measurement of the longitudinal polarization of positrons emitted by polarized 107In nuclei. If interpreted in the framework of the manifest left-right symmetric model, preliminary results yield a lower limit of 303 GeV/c2 for the mass of a possible, predominantly right-handed W gauge boson.

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Improved Treatment Of Loop Diagrams In Su(3) Baryon Chiral Perturbation Theory, Jf Donoghue, Br Holstein Jan 1998

Improved Treatment Of Loop Diagrams In Su(3) Baryon Chiral Perturbation Theory, Jf Donoghue, Br Holstein

Physics Department Faculty Publication Series

Baryon chiral perturbation theory as conventionally applied has a well-known problem with the SU(3) chiral expansion in that loop diagrams generate very large SU(3) breaking corrections and greatly upset the subsequent phenomenology. We argue that the problem comes from the portions of loop integrals corresponding to propagation at such short distances that the effective theory is not valid. One can reformulate the theory equally rigorously by regulating the loop integrals using a momentum-space cutoff which removes the spurious short-distance physics. We show that this reformulation improves the phenomenology for reasonable values of the cutoff.

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Weight For Random Quark Masses, Jf Donoghue Jan 1998

Weight For Random Quark Masses, Jf Donoghue

Physics Department Faculty Publication Series

In theories in which the parameters of the low energy theory are not unique, perhaps having different values in different domains of the universe as is possible in some inflationary models, the fermion masses would be distributed with respect to some weight. In such a situation the specifics of the fermion masses do not have a unique explanation, yet the weight provides the visible remnant of the structure of the underlying theory. This paper introduces this concept of a weight for the distribution of masses and provides a quantitative estimate of it from the observed quarks and leptons. The weight …

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Two-Loop Analysis Of Axial Vector Current Propagators In Chiral Perturbation Theory, E Golowich, J Kambor Jan 1998

Two-Loop Analysis Of Axial Vector Current Propagators In Chiral Perturbation Theory, E Golowich, J Kambor

Physics Department Faculty Publication Series

We perform a calculation of the isospin and hypercharge axial vector current propagators [ΔA3μν(q) and ΔA8μν(q)] to two loops in SU(3)×SU(3) chiral perturbation theory. A large number of O(p6) divergent counterterms are fixed, and complete two-loop renormalized expressions for the pion and eta masses and decay constants are obtained. The calculated isospin and hypercharge axial vector polarization functions are used as input in new chiral sum rules, valid to second order in the light quark masses. Some phenomenological implications of these sum rules are considered.

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Nucleon Axial Matrix Elements, Br Holstein Jan 1998

Nucleon Axial Matrix Elements, Br Holstein

Physics Department Faculty Publication Series

Current issues associated with nucleon axial matrix elements are studied, including the Goldberger-Treiman discrepancy, the induced pseudoscalar, and SU(3) chiral perturbation theory.

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Is Su(3) Chiral Perturbation Theory An Effective Field Theory?, Br Holstein Jan 1998

Is Su(3) Chiral Perturbation Theory An Effective Field Theory?, Br Holstein

Physics Department Faculty Publication Series

We argue that the difficulties associated with the convergence properties of conventional SU(3) chiral perturbation theory can be ameliorated by use of a cutoff, which suppresses the model-dependent short distance effects in such calculations.

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Bonnet Pairs And Isothermic Surfaces, G Kamberov, F Pedit, U Pinkall Jan 1998

Bonnet Pairs And Isothermic Surfaces, G Kamberov, F Pedit, U Pinkall

Mathematics and Statistics Department Faculty Publication Series

In this note we classify all Bonnet pairs on a simply connected domain. Our main intent was to apply what we call a quaternionic function theory to a concrete problem in differential geometry. The ideas are simple: conformal immersions into quaternions or imaginary quaternions take the place of chart maps for a Riemann surface. Starting from a reference immersion we construct all conformal immersions of a given (simply connected) Riemann surface (up to translational periods) by spin transformations. With this viewpoint in mind we discuss how to construct all Bonnet pairs on a simply connected domain from isothermic surfaces and …

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Very Effective String Model?, David Kastor, Jennie Traschen Jan 1998

Very Effective String Model?, David Kastor, Jennie Traschen

Physics Department Faculty Publication Series

Additional evidence is presented for a recently proposed effective string model, conjectured to hold throughout the parameter space of the basic 5 dimensional, triply charged black holes, which includes the effects of brane excitations, as well as momentum modes. We compute the low energy spacetime absorption coefficient σ for the scattering of a triply charged scalar field in the near extremal case, and conjecture an exact form for σ. It is shown that this form of σ arises simply from the effective string model. This agreement encompasses both statistical factors coming from the Bose distributions of string excitations and a …

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Afs And Hac: Domain-General Agent Simulation And Control, Marc S. Atkin, David L. Westbrook, Paul R. Cohen, Gregory D. Jorstad Jan 1998

Afs And Hac: Domain-General Agent Simulation And Control, Marc S. Atkin, David L. Westbrook, Paul R. Cohen, Gregory D. Jorstad

Computer Science Department Faculty Publication Series

We present two systems for simulating and designing agents, respectively. The first, the Abstract Force Simulator (AFS), is a domain-general simulator of agents applying forces; many domains can be characterized in this way. The second, Hierarchical Agent Control (HAC), is a general toolkit for designing an action hierarchy. It supports action abstraction, a multi-level computational architecture, sensor integration, and planning. It is particularly well suited to controlling large numbers of agents in dynamic environments. Together, AFS and HAC provide a very general framework for designing and testing agents.

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Maps For Verbs, Paul Cohen Jan 1998

Maps For Verbs, Paul Cohen

Computer Science Department Faculty Publication Series

This paper describes a representation of the meanings of verbs based on the dynamics of interactions between two agents or objects. The representation treats interactions as having three phases, before, during and after contact. Maps for these phases are constructed. Trajectories through these maps correspond to different types of interactions and are denoted by different verbs. We summarize the results of experiments on learning and reasoning with maps.

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Feature Detection And Identification Using A Sonar-Array, E. G. Araujo Jan 1998

Feature Detection And Identification Using A Sonar-Array, E. G. Araujo

Computer Science Department Faculty Publication Series

This work explores techniques for sonar sensor fusion in the context of environmental feature detection and identification for navigation tasks. By detecting common features in indoor environments and using them as landmarks, a robot can navigate reliably, recovering its pose when necessary. Preliminary results on a multiple hypothesis testing procedure for feature localization and identification show that accurate feature information can be acquired with adequate sonar models and configurations. In addition, a method that associates sonar configuration with the precision of feature extraction is discussed, as well as its utility for guiding an active sonar sensor.

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