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Browsing by Author "Roper, L. David"

Now showing 1 - 20 of 23
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    An analysis of K⁺-nucleon scattering
    Hyslop, John S. (Virginia Tech, 1990-08-15)
    A partial-wave analysis of K⁺-nucleon scattering has been performed. Energy-dependent and energy-independent solutions for the isoscalar and isovector amplitudes are generated by fitting data with a chi-squared minimization technique. The isoscalar amplitudes extend to a K⁺ incident lab kinetic energy of 1100 MeV; the isovector amplitudes extend to 2650 MeV. Due to the lack of a neutron target and scarcity of isoscalar data, K⁺-deuteron inelastic and elastic data are utilized in the analysis. The theories which incorporate the K⁺-deuteron data are fixed-scatterer, single-scattering impulse approximations. Two different techniques are employed to find preliminary energy-dependent isoscalar solutions for the scattering amplitudes. The first technique involves initializing to two different single-energy solutions of previous studies. The second technique consists of utilizing the energy-dependent parametrization to fit successively larger bins of data, starting from zero energy, until the entire energy range is covered. Two solutions result from these investigations since one of the solutions from the first technique agrees with the solution from the second technique. The partial-wave amplitudes are discussed. Resonance pole positions and scattering lengths are extracted from each solution and compared with predictions from theories and other analyses. Also, observables from each solution are compared and experiments are suggested to further refine the determination of the K⁺N partial-wave amplitudes.
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    An analysis of pion photoproduction
    Li, Zhujun L. (Virginia Tech, 1992-04-05)
    A partial-wave analysis of pion photoproduction data up to a photon lab energy of 1.8 GeV has been performed. Both energy-dependent and energy-independent solutions have been obtained. The energy-dependent parametrization incorporates the recently determined elastic pion nucleon scattering amplitudes in such a way as to satisfy unitarity and utilize the resonance structure contained in the pion nucleon elastic amplitudes. Starting from the energy-dependent solution, energy-independent partial-wave solutions are obtained at a set of energies from threshold to 1.8 GeV. The data base used in the analysis contains 11,911 data from the reactions. The predictions of our solution are compared with the experimental data and previous analyses. Suggestions are made for future experiments. A total of sixteen resonances exist in the energy range from threshold to 1.8 GeV. These resonance states are studied using our energy-independent solutions. Photon decay couplings to the sixteen resonances are extracted. These couplings are also compared with previous solutions and quark model predictions.
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    An analysis of pion-nucleon scattering
    Ford, John Marshall (Virginia Polytechnic Institute and State University, 1988)
    A phase-shift analysis of elastic pion-nucleon scattering data g from threshold to a pion kinetic energy of 1100 MeV was performed. The resulting partial-wave amplitudes were investigated in the complex energy plane, and the resonance states with their associated zeros and poles were determined. Particular emphasis was given on elucidating the nature of the P₁₁ partial wave. The phase-shift analysis consisted of both energy-independent and energy-dependent analyses. The energy dependent partial waves were parametrized as a coupled channel K-matrix whose elements are polynomials in energy plus an explicit pole term. A complete description of the investigation and the experimental data used are included as is a description of the theoretical models used for interpretation of the results.
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    Application of classical non-linear Liouville dynamic approximations
    Harter, Terry Lee (Virginia Polytechnic Institute and State University, 1988)
    This dissertation examines the application of the Liouville operator to problems in classical mechanics. An approximation scheme or methodology is sought that would allow the calculation of the position and momentum of an object at a specified later time, given the initial values of the object's position and momentum at some specified earlier time. The approximation scheme utilizes matrix techniques to represent the Liouville operator. An approximation scheme using the Liouville operator is formulated and applied to several simple one-dimensional physical problems, whose solution is obtainable in terms of known analytic functions. The scheme is shown to be extendable relative to cross products and powers of the variables involved. The approximation scheme is applied to a more complicated one-dimensional problem, a quartic perturbed simple harmonic oscillator, whose solution is not capable of being expressed in terms of simple analytic functions. Data produced by the application of the approximation scheme to the perturbed quartic harmonic oscillator is analyzed statistically and graphically. The scheme is reapplied to the solution of the same problem with the incorporation of a drag term, and the results analyzed. The scheme is then applied to a simple physical pendulum having a functionalized potential in order to ascertain the limits of the approximation technique. The approximation scheme is next applied to a two-dimensional non-perturbed Kepler problem. The data produced is analyzed statistically and graphically. Conclusions are drawn and suggestions are made in order to continue the research in several of the areas presented.
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    An application of the Liouville resolvent method to the study of fermion-boson couplings
    Bressler, Barry Lee (Virginia Polytechnic Institute and State University, 1986)
    The Liouville resolvent method is an unconventional technique used for finding a Green function for a Hamiltonian. Implementation of the method entails the calculation of commutators of a second-quantized Hamiltonian operator with particular generalized stepping operators that are elements of a Hilbert space and that represent transitions between many-particle states. These commutators produce linear combinations of stepping operators, so the results can be arrayed as matrix elements of the Liouville operator L̂ in the Hilbert space of stepping operators. The resulting L̂ matrix is usually of infinite order, and in principle its eigenvalues and eigenvectors can be used to construct the Green function from the L̂ resolvent matrix. Approximations are usually necessary, at least in the form of truncation of the L̂ matrix, and if one produces a sequence of such matrices of increasing order and calculates the eigenvalues and eigenvectors of these matrices, a sequence of approximations for the L̂ resolvent matrix can be produced. This sequence is mathematically guaranteed to converge to the exact result for the L̂ resolvent matrix (except at its singularities). The accuracy of an approximation depends on the order of the matrix at which the sequence is truncated. Application of the method to a Hamiltonian representing interactions between fermions and bosons involves complications arising from the large number of terms generated by the commutation properties of boson operators. This dissertation describes the method and its use in the study of fermion-boson couplings. Approximations to second order in stepping operators are calculated for simplified Froehlich and Lee models. Limited thermodynamic results are obtained from the Lee model. Exact energy eigenvalues are obtained by operator algebra for simplified Froehlich, Lee and Dirac models. These exact solutions comprise the main contribution of this research and will prove to be valuable starting points for further research. Suggestions are made for further research.
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    A Conversation with S. N. Postlethwait
    Postlethwait, Samuel; Terrell, W. R.; Roper, L. David; Braden, Roberts A. (Virginia Tech. Learning Resources Center, 1978-08-08)
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    Explicitly structured physics instruction
    Wright, David Shaw (Virginia Polytechnic Institute and State University, 1984)
    In an introductory physics course, problem solving skills are not traditionally taught. The instructor explains the physical theory and works example problems. Many students, however, are not able to develop the ability to solve problems implicitly. The program of Explicitly Structured Physics Instruction (ESPI) was developed to teach problem solving skills explicitly. It is designed to help students organize their work, increase their accuracy, eliminate initial panic or lack of direction in approaching a problem, increase confidence in problem solving, promote understanding instead of rote memory, and improve the students' ability to communicate with the instructor and other students. It provides not only an explicit strategy for problem solving, but also a structure for examining formulas called the formula fact sheet, and an opportunity for practice and feedback in a problem solving session which involves the use of out loud thinking. The program of ESPI was developed over five academic quarters of testing. A statistical analysis was performed on the data obtained, but the qualitative data obtained from student interviews and questionnaires, as well as the instructor reaction to the program, provided the main source of input in the development of the program and the measurement of its success. Reaction to the program in its final revised form was very positive. Over 90% said that they would use the strategy even if it were not required, and that the formula fact sheet had been very helpful. Over 75% said that the problem solving session was very helpful. Final grades of those who used the strategy were significantly higher than those who did not. Retention of students in the course was raised from 70% to 86%. The study indicates that a well integrated program built around the use of a problem solving strategy can help students focus on understanding physics and the problem sovling process.
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    Fermions in Yang-Mills gauge theories: invariance, covariance and topology
    Liang, Yigao (Virginia Polytechnic Institute and State University, 1987)
    I present a study on the invariance and covariance properties of the Dirac operator describing fermions in Yang-Mills fields. This includes the study of anomalies of the gauge currents. We are particularly interested in the geometric and topological features in the problem. The complicated topological structures and properties present in these theories are made clear by elementary calculations in several simple models. We show explicitly how non-trivial phase and sign ambiguities arise to give the so-called anomalies. The Atiyah-Singer index theorem is seen to be a very powerful tool to calculate the topological invariants that characterize the anomalies. The index theorem also gives topological invariants describing the failure of covariance of the fermion propagator.
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    Inclusive resonance production in single-vee events in [pi]- nucleon interactions at 200 GeV
    Mikocki, Stanislaw (Virginia Polytechnic Institute and State University, 1985)
    We present results from Fermilab MPS experiment E580 on the reactions π⁻N->V°X where V° is K°S, A or A̅ and X are charged particles. Transverse and longitudinal momentum distributions for the V° and for K*±(892), Σ±(1385), Ξ⁻(1321) and Ξ⁺(1321) are presented. A comparison is made with the prediction of QCD quark counting rules. Evidence for A⁺C; is presented.
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    Monte Carlo simulation of aqueous dilute solutions of polyhydric alcohols
    Lilly, Arnys Clifton (Virginia Polytechnic Institute and State University, 1989)
    In order to investigate the details of hydrogen bonding and solution molecular conformation of complex alcohols in water, isobaric-isothermal Monte Carlo simulations were carried out on several systems. The solutes investigated were ethanol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol and glycerol. In addition, propane, which does not hydrogen bond but does form water hydrates, was simulated in aqueous solution. The complex alcohol-water systems are very nonideal in their behavior as a function of solute concentration down to very dilute solutions. The water model employed was TIP4P water¹ and the intermolecular potentials employed are of the Jorgensen type² in which the interactions between the molecules are represented by interaction sites usually located on nuclei. The interactions are represented by a sum of Coulomb and Lennard-Jones terms between all intermolecular pairs of sites. Intramolecular rotations in the solute are modeled by torsional potential energy functions taken from ethanol, 1-propanol and 2-propanol for C-O and C-C bond rotations. Quasi-component pair correlation functions were used to analyze the hydrogen bonding. Hydrogen bonds were classified as proton acceptor and proton donor bonds by analyzing the nearest neighbor pair correlation function between hydroxyl oxygen and hydrogen and between solvent-water hydrogen and oxygen. The results obtained for partial molar heats of solution are more negative than the experimental values by 3.0 to 14 kcal/mol. In solution, all solutes reached a contracted molecular geometry with the OH groups generally on one side of the molecule. There is a tendency for the solute OH groups to hydrogen bond with water, with more proton acceptor bonds than proton donor bonds. The water-solute binding energies correlate with experimental measurements of the water-binding properties of the solute. 1. Jorgensen, W.L. et al, J. Chem. Phys., 79, 926 (1983). 2. Jorgensen, W.L., J. Phys Chem., 87, 5304 (1983).
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    Multiple reflector scanning antennas
    Shen, Bing (Virginia Tech, 1993)
    Narrow beamwidth antenna systems are important to remote sensing applications and point-to-point communication systems. In many applications the main beam of the antenna radiation pattern must be scannable over a region of space. Scanning by mechanically skewing the entire antenna assembly is difficult and in many situations is unacceptable. Performance during scan is, of course, also very important. Traditional reflector systems employing the well-focused paraboloidal-shaped main reflector accomplish scan by motion of a few feeds, or by phase steering a focal plane feed array. Such scanning systems can experience significant gain loss. Traditional reflecting systems with a spherical main reflector have low aperture efficiency and poor side lobe and cross polarization performance. This dissertation introduces a new approach to the design of scanning spherical reflector systems, in which the performance weaknesses of high cross polarization and high side lobe levels are avoided. Moreover, the low aperture utilization common in spherical reflectors is overcome. As an improvement to this new spherical main reflector configuration, a flat mirror reflector is introduced to minimize the mechanical difficulties to scan the main beam. In addition to the reflector system design, reflector antenna performance evaluation is also important. The temperature resolution issue important for earth observation radiometer antennas is studied, and a new method to evaluate and optimize such temperature resolution is introduced.
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    Nonlinear optical effects in nematic liquid crystals
    Puang-ngern, Srisuda (Virginia Polytechnic Institute and State University, 1985)
    Theoretical studies of nonlinear optical effects in nematic liquid crystals including degenerate four-wave mixing are presented. The optically induced Freedericksz transition which is essential for these effects is also described. Experimental investigations are performed using a homeotropically aligned MBBA thin film. Good agreement is obtained between the theoretical predictions and the experiments. Some potential applications of phase conjugation obtained by the backward degenerate four wave-mixing process in the field of adaptive optics and image processing are demonstrated.
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    Observation of K* (2060) in the [phi]K[pi] final state produced in proton-nuclei interactions at 400 GeV/c
    Torres, Sergio (Virginia Polytechnic Institute and State University, 1985)
    In 1982 Fermilab experiment E623 took data in a search for high-mass boson resonances which decay into one or two ɸ mesons produced in 400 GeV/c proton-nuclei interactions. A hardware trigger selected 3.6 million events with identified K⁺K⁻ pairs. Here I report my study of events with ɸKπ in the final state and present the first evidence for K°*(2060) decaying into ɸKπ and ɸK°*(890).
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    A Panel Discussion with Fred S. Keller
    Keller, Fred S.; Sherman, Thomas M.; Roper, L. David; Braden, Roberts A. (Virginia Tech. Learning Resources Center, 1979-05)
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    Perturbative QCD in exclusive processes
    Zhang, Huayi (Virginia Polytechnic Institute and State University, 1987)
    A computer program that symbolically generates and evaluates all Feynman diagrams required for scattering amplitude for exclusive processes is tested, corrected, extended, and brought to operational status. The sensitivity of perturbative QCD predictions for the nucleon form factors, ψ → pp̅, and 𝛾𝛾 → pp̅, to the theoretical uncertainties of the nucleon wave function and the form of the running coupling constant is investigated. A new prediction for the cross-section for 𝛾𝛾 → Δ++ Δ̅++ with sum-rule wave functions is presented. As a product of the development of the computer program, the quark amplitudes for meson-baryon scattering are obtained. Integrations of the quark amplitudes over wave functions are carried out by cutting off singularities. The numerical reliability of the integration and its sensitivity to the cut-off’s and the choice of wave function are investigated.
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    Possibility of positive-pulse switching in systems of nonlinear Fabry-Perot cavities
    Ho, Kwongchoi Caisy (Virginia Tech, 1991-12-05)
    The conventional way of using a nonlinear Fabry-Perot cavity as an optical memory requires a negative pulse input to reset the state of the cavity. The possibility of using positive pulses to turn a system of nonlinear Fabry-Perot cavities on and off is studied and it was found that positive pulse switching is possible in a system of two coupled nonlinear cavities. First, Korpel and Lohmann's proposal of using polarization switching in a single nonlinear birefringent cavity was studied. After a detailed investigation of their proposal it was found that positive pulse switching in a single nonlinear Fabry-Perot cavity is not possible. One of the reasons is that the eigen-polarization states of the output of a nonlinear Fabry-Perot cavity cannot be switched independently. Although it is not possible to switch a single nonlinear Fabry-Perot cavity with positive pulses we were able to use the coupling of the eigen-polarization states to implement other kinds of optical switches which were demonstrated experimentally. The cross-talk effect in a metallic Fabry-Perot cavity was also studied. Next, a steady state model of a system of two coupled nonlinear Fabry-Perot cavities was developed and it was found that positive pulse switching is possible in such a system. The output can be turned on and off either by pulses sent into different cavities or by pulses of different magnitudes sent into one cavity. Finally, the dynamic behavior of the coupled cavities system was modeled by extending Goldstone and Garmire's model of a single cavity with one input to a system of two coupled cavities with two inputs. We verified by numerical calculations that positive pulse switching is also possible in the dynamic regime.
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    Proton Compton scattering with polarized γ rays
    Zhao, Xi-jun (Virginia Tech, 1993)
    Proton Compton scattering has been studied from 220 to 330 MeV at 90 degree center of mass scattering angle with the polarized photon beam at LEGS. Compton scattering is an important probe of the nucleon structure and its excited states. Polarization degrees of freedom reveal more information than unpolarized observables. This experiment measured, for the first time, the polarized cross sections of proton Compton scattering up to the Δ resonance. The parallel and perpendicular cross sections were measured at the same time. All the photons were tagged so that the energy dependent systematic errors are small. The measured unpolarized cross section is above the lower bound from unitary at all energies. The cross section asymmetry is obtained as a function of energy for the first time. The unpolarized cross section is compared with previous data and with calculations from the fixed-t dispersion model, the isobar model and the finite energy dispersion model. Although all these calculations agree fairly well with unpolarized cross section data, the photon asymmetry data show that the isobar model contains serious defects. The ratio of polarized cross sections, dσ⫫/dσ⟂, is compared with a model independent result. The results from the present experiment can be used in the calculation of the E2/M1 ratio in the N — Δ transition, which is an important signature of the tensor interaction between quarks.
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    Rapidity gap in the final state hadron distribution in deep inelastic electron scattering at HERA
    Blankenship, Thomas Keith (Virginia Tech, 1995-02-03)
    The rapidity space available for hadron production in electron-proton interactions at HERA is 11.4 units in width. In such a space the separation of virtual photon fragmentation from proton fragmentation is observed in events where the invariant mass of the final state hadronic system, W, is greater than 130 Ge V. Experimental cross sections for the virtual photon fragmentation process are presented. A new class of events is identified, where only virtual photon fragmentation and no proton fragmentation is observed. In the final state hadronic system of these events a rapidity gap over four units in width was discovered. Cross sections for these events are also presented, and comparisons are made with the theoretical predictions of the pomeron and hard diffraction models.
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    A statistical theory of the epilepsies
    Thomas, Kuryan (Virginia Polytechnic Institute and State University, 1988)
    A new physical and mathematical model for the epilepsies is proposed, based on the theory of bond percolation on finite lattices. Within this model, the onset of seizures in the brain is identified with the appearance of spanning clusters of neurons engaged in the spurious and uncontrollable electrical activity characteristic of seizures. It is proposed that the fraction of excitatory to inhibitory synapses can be identified with a bond probability, and that the bond probability is a randomly varying quantity displaying Gaussian statistics. The consequences of the proposed model to the treatment of the epilepsies is explored. The nature of the data on the epilepsies which can be acquired in a clinical setting is described. It is shown that such data can be analyzed to provide preliminary support for the bond percolation hypothesis, and to quantify the efficacy of anti-epileptic drugs in a treatment program. The results of a battery of statistical tests on seizure distributions are discussed. The physical theory of the electroencephalogram (EEG) is described, and extant models of the electrical activity measured by the EEG are discussed, with an emphasis on their physical behavior. A proposal is made to explain the difference between the power spectra of electrical activity measured with cranial probes and with the EEG. Statistical tests on the characteristic EEG manifestations of epileptic activity are conducted, and their results described. Computer simulations of a correlated bond percolating system are constructed. It is shown that the statistical properties of the results of such a simulation are strongly suggestive of the statistical properties of clinical data. The study finds no contradictions between the predictions of the bond percolation model and the observed properties of the available data. Suggestions are made for further research and for techniques based on the proposed model which may be used for tuning the effects of anti-epileptic drugs.
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    Studies of lepton and quark interactions
    Wang, Ping (Virginia Polytechnic Institute and State University, 1985)
    Part I Proposed Experimental Tests of the Right-handed Weak Current All possible experiments which test the SU(2)L x U(1)R x U(1)B-L model and SU(2)L x SU(2)R x U(1)B-L model using the LEP e⁺e⁻ collider and HERA e⁻p collider are calculated and the most sensitive experiments are examined. Part II Semi-Phenomenological Theory of (Qq̅) System The (QQ̅) and (Qq̅) mesons are calculated using a QCD motivated potential model. It is discovered that by including a long distance relativistic correction term derived by Grome, the Coulomb + Linear potential works not only for c and b quarks, but s quark as well. The leptonic decay constants of various (Qq̅) mesons together with their masses are predicted. The topponium states are also discussed.