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2001

Series

University of Massachusetts Amherst

Articles 1 - 30 of 71

Full-Text Articles in Physical Sciences and Mathematics

Institutsdagen At The Royal Institute Of Technology, Stockholm, Sweden, May 3, 2000, Otto Vogl, Ann Christine Albertsson Dec 2001

Institutsdagen At The Royal Institute Of Technology, Stockholm, Sweden, May 3, 2000, Otto Vogl, Ann Christine Albertsson

Emeritus Faculty Author Gallery

No abstract provided.

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Poly Millennial 2000 Hilton Waikoloa Villages, Hawaii, December 9-13, 2000, Otto Vogl, William Daly Nov 2001

Poly Millennial 2000 Hilton Waikoloa Villages, Hawaii, December 9-13, 2000, Otto Vogl, William Daly

Emeritus Faculty Author Gallery

No abstract provided.

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Investigating The Long-Term Evolution Of Galaxies: Noise, Cuspy Halos And Bars, Martin D. Weinberg Oct 2001

Investigating The Long-Term Evolution Of Galaxies: Noise, Cuspy Halos And Bars, Martin D. Weinberg

Astronomy Department Faculty Publication Series

I review the arguments for the importance of halo structure in driving galaxy evolution and coupling a galaxy to its environment. We begin with a general discussion of the key dynamics and examples of structure dominated by modes. We find that simulations with large numbers of particles (N ≥ 106) are required to resolve the dynamics. Finally, I will describe some new results which demonstrates that a disk bar can produce cores in a cuspy CDM dark-matter profile within a gigayear. An inner Lindblad-like resonance couples the rotating bar to halo orbits at all radii through the cusp, rapidly flattening …

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A Polytope Combinatorics For Semisimple Groups, Jared E. Anderson Oct 2001

A Polytope Combinatorics For Semisimple Groups, Jared E. Anderson

Mathematics and Statistics Department Faculty Publication Series

Mirkovi and Vilonen discovered a canonical basis of algebraic cycles for the intersection homology of (the closures of the strata of) the loop Grassmannian. The moment map images of these varieties are a collection of polytopes, and they may be used to compute weight multiplicities and tensor product multiplicities for representations of a semisimple group. The polytopes are explicitly described for a few low rank groups.

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Language Models For Hierarchical Summarization (Proposal For Dissertation), Dawn Lawrie May 2001

Language Models For Hierarchical Summarization (Proposal For Dissertation), Dawn Lawrie

Computer Science Department Faculty Publication Series

Hierarchies have long been used for organization, summarization, and access to information. In this proposal we define summarization in terms of a probabilistic language model and use the definition to explore new techniques for automatically generating topic hierarchies. One technique applies a graph-theoretic algorithm, which is an approximation of the Dominating Set Problem. Another technique uses an entropy-based approach to choose topic terms. Both techniques efficiently select terms according to a language model. We compare the new techniques to previous methods proposed for constructing topic hierarchies including subsumption and lexical hierarchies, as well as words found using TF.IDF. Our preliminary …

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Quaternionic Holomorphic Geometry: Plucker Formula, Dirac Eigenvalue Estimates And Energy Estimates Of Harmonic 2-Tori, D Ferus, K Leschke, F Pedit, U Pinkall Jan 2001

Quaternionic Holomorphic Geometry: Plucker Formula, Dirac Eigenvalue Estimates And Energy Estimates Of Harmonic 2-Tori, D Ferus, K Leschke, F Pedit, U Pinkall

Mathematics and Statistics Department Faculty Publication Series

No abstract provided.

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Geometric Representation Theory Of Restricted Lie Algebras, I Mirkovic, D Rumynin Jan 2001

Geometric Representation Theory Of Restricted Lie Algebras, I Mirkovic, D Rumynin

Mathematics and Statistics Department Faculty Publication Series

We modify the Hochschild -map to construct central extensions of a restricted Lie algebra. Such central extension gives rise to a goup scheme that leads to a geometric construction of unrestricted representations. For a classical semisimple Lie algebra, we construct equivariant line bundles whose global sections afford representations with a nilpotentp-character.

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4. Bengt Rånby (1920-2000), Otto Vogl, Gerald S. Kirshenbaum Jan 2001

4. Bengt Rånby (1920-2000), Otto Vogl, Gerald S. Kirshenbaum

Emeritus Faculty Author Gallery

No abstract provided.

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Department Of Physics Newsletter: Spring 2001, Gerry Peterson, Bob Krotkov, Mary Ann Ryan, Kristine Reopell Jan 2001

Department Of Physics Newsletter: Spring 2001, Gerry Peterson, Bob Krotkov, Mary Ann Ryan, Kristine Reopell

Physics Newsletters

No abstract provided.

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On-Line Matrix Removal Of Lead For The Determination Of Trace Elements In Forensic Bullet Samples By Flow Injection Inductively Coupled Plasma-Mass Spectrometry, Er Yourd, Jf Tyson, Rd Koons Jan 2001

On-Line Matrix Removal Of Lead For The Determination Of Trace Elements In Forensic Bullet Samples By Flow Injection Inductively Coupled Plasma-Mass Spectrometry, Er Yourd, Jf Tyson, Rd Koons

Chemistry Department Faculty Publication Series

No abstract provided.

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Microwave-Assisted Extraction Of Monomethyl Arsonic Acid From Soil And Sediment Standard Reference Materials, Pm Yehl, H Gurleyuk, Jf Tyson, Pc Uden Jan 2001

Microwave-Assisted Extraction Of Monomethyl Arsonic Acid From Soil And Sediment Standard Reference Materials, Pm Yehl, H Gurleyuk, Jf Tyson, Pc Uden

Chemistry Department Faculty Publication Series

No abstract provided.

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Statistical Equilibrium Measures In Micromagnetics, Ma Katsoulakis, P Plechac Jan 2001

Statistical Equilibrium Measures In Micromagnetics, Ma Katsoulakis, P Plechac

Mathematics and Statistics Department Faculty Publication Series

We derive an equilibrium statistical theory for the macroscopic description of a ferromagnetic material at positive finite temperatures. Our formulation describes the most-probable equilibrium macrostates that yield a coherent deterministic large-scale picture varying at the size of the domain, as well as it captures the effect of random spin fluctuations caused by the thermal noise. We discuss connections of the proposed formulation to the Landau-Lifschitz theory and to the studies of domain formation based on Monte Carlo lattice simulations.

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Phenomenological Issues In The Determination Of Delta Gamma(D), E Golowich, S Pakvasa Jan 2001

Phenomenological Issues In The Determination Of Delta Gamma(D), E Golowich, S Pakvasa

Physics Department Faculty Publication Series

We consider the issue of determining the D0– width difference ΔΓD experimentally. The current situation is reviewed and suggestions for further study are given. We propose a number of D0 decay modes in addition to those studied in the recent E791, FOCUS and BELLE lifetime determination experiments. Then we address prospects for determining CF-CDS strong phase differences, like δKπ which appears in the CLEO study of D0→K+π− transitions. We show how to extract δK*π with CDS data and furthermore show when D→KLπ data becomes available that δKπ can also be obtained.

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Worm Algorithms For Classical Statistical Models, Nikolai Prokof'ev, Boris Svistunov Jan 2001

Worm Algorithms For Classical Statistical Models, Nikolai Prokof'ev, Boris Svistunov

Physics Department Faculty Publication Series

We show that high-temperature expansions provide a basis for the novel approach to efficient Monte Carlo simulations. “Worm” algorithms utilize the idea of updating closed-path configurations (produced by high-temperature expansions) through the motion of end points of a disconnected path. An amazing result is that local, Metropolis-type schemes using this approach appear to have dynamical critical exponents close to zero (i.e., their efficiency is comparable to the best cluster methods) as proved by finite-size scaling of the autocorrelation time for various universality classes.

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Single-Hole Spectral Function And Spin-Charge Separation In The T-J Model, A Mishchenko, Nikolai Prokof'ev, Boris Svistunov Jan 2001

Single-Hole Spectral Function And Spin-Charge Separation In The T-J Model, A Mishchenko, Nikolai Prokof'ev, Boris Svistunov

Physics Department Faculty Publication Series

Worm algorithm Monte Carlo simulations of the hole Green function with subsequent spectral analysis were performed for 0.1<~J/t<~0.4 on lattices with up to L×L=32×32 sites at a temperature as low as T=J/40, and present, apparently, the hole spectral function in the thermodynamic limit. Spectral analysis reveals a δ-function-sharp quasiparticle peak at the lower edge of the spectrum that is incompatible with the power-law singularity and thus rules out the possibility of spin-charge separation in this parameter range. Spectral continuum features two peaks separated by a gap ∼4÷5 t.

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Diagrammatic Quantum Monte Carlo For Two-Body Problems: Applied To Excitons, E Burovski, A Mishchenko, Nikolai Prokof'ev, B Svistunov Jan 2001

Diagrammatic Quantum Monte Carlo For Two-Body Problems: Applied To Excitons, E Burovski, A Mishchenko, Nikolai Prokof'ev, B Svistunov

Physics Department Faculty Publication Series

We present a novel method for the precise numeric solution of the irreducible two-body problem and apply it to excitons in solids. The approach is based on the Monte Carlo simulation of the two-body Green function specified by Feynman’s diagrammatic expansion. Our method does not rely on the specific form of the electron and hole dispersion laws and is valid for any attractive electron-hole potential. We establish limits of validity of the Wannier (large radius) and Frenkel (small radius) approximations, present accurate data for the intermediate radius excitons, and give evidence for the charge transfer nature of the monopolar exciton …

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Calibrations And Fayyazuddin-Smith Spacetimes, Hyunji Cho, Moataz Emam, David Kastor, Jennie Traschen Jan 2001

Calibrations And Fayyazuddin-Smith Spacetimes, Hyunji Cho, Moataz Emam, David Kastor, Jennie Traschen

Physics Department Faculty Publication Series

We show that a class of spacetimes introduced by Fayyazuddin and Smith to describe intersecting M5-branes admits a generalized Kähler calibration. Equipped with this understanding, we are able to construct spacetimes corresponding to further classes of calibrated p-brane world-volume solitons. We note that these classes of spacetimes also describe the fields of p-branes wrapping certain supersymmetric cycles of Calabi-Yau manifolds.

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Subleading Corrections To Parity-Violating Pion Photoproduction, Sl Zhu, S Puglia, Br Holstein, Mj Ramsey-Musolf Jan 2001

Subleading Corrections To Parity-Violating Pion Photoproduction, Sl Zhu, S Puglia, Br Holstein, Mj Ramsey-Musolf

Physics Department Faculty Publication Series

We compute the photon asymmetry Bγ for near threshold parity-violating (PV) pion photoproduction through subleading order. We show that subleading contributions involve a new combination of PV couplings not included in previous analyses of hadronic PV. We argue that existing constraints on the leading order contribution to Bγ—obtained from the PV γ-decay of 18F—suggest that the impact of the subleading contributions may be more significant than expected from naturalness arguments.

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Introduction To Effective Field Theory, Br Holstein Jan 2001

Introduction To Effective Field Theory, Br Holstein

Physics Department Faculty Publication Series

This review summarizes effective field theory techniques, which are the modern theoretical tools for exploiting the existence of hierarchies of scale in a physical problem. The general theoretical framework is described and evaluated explicitly for a simple model. Power-counting results are illustrated for a few cases of practical interest, and several applications to quantum electrodynamics are described.

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Chiral Symmetry And The Parity-Violating Nn Pi Yukawa Coupling, Sl Zhu, Sj Puglia, Br Holstein, Mj Ramsey-Musolf Jan 2001

Chiral Symmetry And The Parity-Violating Nn Pi Yukawa Coupling, Sl Zhu, Sj Puglia, Br Holstein, Mj Ramsey-Musolf

Physics Department Faculty Publication Series

We construct the complete SU(2) parity-violating (PV) π,N,Δ interaction Lagrangian with one derivative, and calculate the chiral corrections to the PV Yukawa NNπ coupling constant hπ through O(1/Λχ3) in the leading order of heavy baryon expansion. We discuss the relationship between the renormalized hπ, the measured value of hπ, and the corresponding quantity calculated microscopically from the standard model four-quark PV interaction. We observe that the renormalized hπ depends strongly on a number of a priori unknown parameters in the PV effective Lagrangian.

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Structure Of The Large Magellanic Cloud From 2mass, Md Weinberg, S Nikolaev Jan 2001

Structure Of The Large Magellanic Cloud From 2mass, Md Weinberg, S Nikolaev

Astronomy Department Faculty Publication Series

We derive structural parameters and evidence for extended tidal debris from star count and preliminary standard candle analyses of the Large Magellanic Cloud based on Two-Micron All-Sky Survey (2MASS) data. The full-sky coverage and low extinction in Ks present an ideal sample for structural analysis of the LMC. The star count surface densities and deprojected inclination for both young and older populations are generally consistent with previous work. We fit the projected density with an exponential disk or spherical power-law model with an optional two-dimensional elliptical bar. The derived disk scale length is R = 1.42 ± 0.01 kpc for …

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Noise-Driven Evolution In Stellar Systems - Ii. A Universal Halo Profile, Md Weinberg Jan 2001

Noise-Driven Evolution In Stellar Systems - Ii. A Universal Halo Profile, Md Weinberg

Astronomy Department Faculty Publication Series

Disc instabilities such as arm and bar formation, minor mergers and tidal encounters drive a galaxy from equilibrium. Using the theory that describes the evolution of a galaxy halo as a result of stochastic fluctuations developed in the companion paper to this one, we show that this sort of noise evolves a halo toward a standard profile, independent of its initial profile and concentration. This process can substantially redistribute the mass in dark-matter haloes in the 10 Gyr since formation. Three different noise processes are studied: (i) a bombardment by blobs of mass that are small compared to the halo …

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Periodic Photometric Variability In The Becklin-Neugebauer Object, Lynne A. Hillenbrand, John M. Carpenter, Michael F. Skrutskie Jan 2001

Periodic Photometric Variability In The Becklin-Neugebauer Object, Lynne A. Hillenbrand, John M. Carpenter, Michael F. Skrutskie

Astronomy Department Faculty Publication Series

The Becklin-Neugebauer (BN) object in the Orion Nebula Cluster (ONC) is a well-studied optically invisible, infrared-bright young stellar object, thought to be an intermediate-mass protostar. We report here that BN exhibited nearly-sinusoidal periodic variability at the near-infrared H- and Ks-bands during a one month observing campaign in 2000 March/April. The period was 8.28 days and the peak-to-peak amplitude ~0.2 mag. Plausible mechanisms for producing the observed variability characteristics are explored.

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Three Wide Separation L-Dwarf Companions From The Two Micron All Sky Survey: Gl 337c, Gl 618.1b, And Hd 89744b, J. C. Wilson, Davy Kirkpatrick, G. E. Gizis, M. F. Skrutskie, D. G. Monet, J. R. Houck Jan 2001

Three Wide Separation L-Dwarf Companions From The Two Micron All Sky Survey: Gl 337c, Gl 618.1b, And Hd 89744b, J. C. Wilson, Davy Kirkpatrick, G. E. Gizis, M. F. Skrutskie, D. G. Monet, J. R. Houck

Astronomy Department Faculty Publication Series

We present two confirmed wide separation L-dwarf common proper motion companions to nearby stars and one candidate identified from the Two Micron All Sky Survey. Spectral types from optical spectroscopy are L0 V, L2.5 V, and L8 V. Near-infrared low resolution spectra of the companions are provided as well as a grid of known objects spanning M6 V -- T dwarfs to support spectral type assignment for these and future L-dwarfs in the z'JHK bands. Using published measurements, we estimate ages of the companions from physical properties of the primaries. These crude ages allow us to estimate companion masses using …

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Near-Infrared Photometric Variability Of Stars Toward The Orion A Molecular Cloud, John M. Carpenter, Lynne A. Hillenbrand, M. F. Skrutskie Jan 2001

Near-Infrared Photometric Variability Of Stars Toward The Orion A Molecular Cloud, John M. Carpenter, Lynne A. Hillenbrand, M. F. Skrutskie

Astronomy Department Faculty Publication Series

We present an analysis of J, H, and Ks time-series photometry obtained with the southern 2MASS telescope over a 084 × 6° region centered near the Trapezium region of the Orion Nebula cluster. These data are used to establish the near-infrared variability properties of pre–main-sequence stars in Orion on timescales of ∼1–36 days, ∼2 months, and ∼2 years. A total of 1235 near-infrared variable stars are identified, ∼93% of which are likely associated with the Orion A molecular cloud. The variable stars exhibit a diversity of photometric behavior with time, including cyclic fluctuations with periods up to 15 days, aperiodic …

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Cormass: A Compact And Efficient Near‐Infrared Spectrograph For Studying Low‐Mass Objects, J. C. Wilson, M. F. Skrutskie, M. R. Colonno, A. T. Enos, J. D. Smith, C. P. Henderson, J. E. Gizis, D. G. Monet, J. R. Houck Jan 2001

Cormass: A Compact And Efficient Near‐Infrared Spectrograph For Studying Low‐Mass Objects, J. C. Wilson, M. F. Skrutskie, M. R. Colonno, A. T. Enos, J. D. Smith, C. P. Henderson, J. E. Gizis, D. G. Monet, J. R. Houck

Astronomy Department Faculty Publication Series

This is a pre-published version which is collected from arXiv. The published version is at http://www.journals.uchicago.edu/doi/abs/10.1086/318619.

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Cooling Radiation And The Ly Alpha Luminosity Of Forming Galaxies, Ma Fardal, N Katz, Jp Gardner, L Hernquist, Dh Weinberg, R Dave Jan 2001

Cooling Radiation And The Ly Alpha Luminosity Of Forming Galaxies, Ma Fardal, N Katz, Jp Gardner, L Hernquist, Dh Weinberg, R Dave

Astronomy Department Faculty Publication Series

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Lyα line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z 2. However, in contrast to the classical picture of gas cooling from the ~106 K …

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Metal Enrichment Of The Intergalactic Medium In Cosmological Simulations, A Aguirre, L Hernquist, J Schaye, N Katz, Dh Weinberg, J Gardner Jan 2001

Metal Enrichment Of The Intergalactic Medium In Cosmological Simulations, A Aguirre, L Hernquist, J Schaye, N Katz, Dh Weinberg, J Gardner

Astronomy Department Faculty Publication Series

Observations have established that the diffuse intergalactic medium (IGM) at z ~ 3 is enriched to ~10-2.5 solar metallicity and that the hot gas in large clusters of galaxies (ICM) is enriched to - Z at z = 0. Metals in the IGM may have been removed from galaxies (in which they presumably form) during dynamical encounters between galaxies, by ram-pressure stripping, by supernova-driven winds, or as radiation-pressure-driven dust efflux. This study develops a method of investigating the chemical enrichment of the IGM and of galaxies, using already completed cosmological simulations. To these simulations we add dust and (gaseous) …

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Hydrodynamic Simulation Of The Cosmological X-Ray Background, Rac Croft, T Di Matteo, L Hernquist, N Katz, Ma Fardal, Dh Weinberg Jan 2001

Hydrodynamic Simulation Of The Cosmological X-Ray Background, Rac Croft, T Di Matteo, L Hernquist, N Katz, Ma Fardal, Dh Weinberg

Astronomy Department Faculty Publication Series

We use a hydrodynamic simulation of an inflationary cold dark matter model with a cosmological constant to predict properties of the extragalactic X-ray background (XRB). We focus on emission from the intergalactic medium (IGM), with particular attention to diffuse emission from warm-hot gas that lies in relatively smooth filamentary structures between galaxies and galaxy clusters. We also include X-rays from point sources associated with galaxies in the simulation, and we make maps of the angular distribution of the emission. Although much of the X-ray luminous gas has a filamentary structure, the filaments are not evident in the simulated maps because …

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Group Scaling Relations From A Cosmological Hydrodynamic Simulation: No Pre-Heating Required?, R Dave, N Katz, L Hernquist, D Weinberg Jan 2001

Group Scaling Relations From A Cosmological Hydrodynamic Simulation: No Pre-Heating Required?, R Dave, N Katz, L Hernquist, D Weinberg

Astronomy Department Faculty Publication Series

We investigate the X-ray vs. optical scaling relations of poor groups to small clusters (sigma~100-700 km/s) identified in a cosmological hydrodynamic simulation of a Lambda-CDM universe, with cooling and star formation but no pre-heating. We find that the scaling relations between X-ray luminosity, X-ray temperature, and velocity dispersion show significant departures from the relations predicted by simple hydrostatic equilibrium models or simulations without cooling, having steeper L_X-sigma and L_X-T_X slopes and a "break" at \~200 km/s (~0.3 keV). These departures arise because the hot (X-ray emitting) gas fraction varies substantially with halo mass in this regime. Our predictions roughly agree …

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