39 Matching Results

Search Results

Advanced search parameters have been applied.

Power corrections, renormalons and resummation

Description: I briefly review three topics of recent interest concerning power corrections, renormalons and Sudakov resummation: (a) 1/Q corrections to event shape observables in e(+)e(-) annihilation, (b) power corrections in Drell-Yan production and (c) factorial divergences that arise in resummation of large infrared (Sudakov) logarithms in moment or `real` space.
Date: August 1, 1996
Creator: Beneke, M.
Partner: UNT Libraries Government Documents Department

Precsion Supersymmetry Measurement at the e{minus} e{minus} Collider

Description: Measurements of supersymmetric particle couplings provide important verification of supersymmetry. If some of the superpartners are at the multi-TeV scale, they will escape direct detection at planned future colliders. However, such particles induce nondecoupling corrections in processes involving the accessible superparticles through violations of the supersymmetric equivalence between gauge boson and gaugino couplings. These violations are analogous to the oblique corrections in the electroweak sector of the standard model, and can be parametrized in terms of super-oblique parameters. The e{sup -}e{sup -} collision mode of a future linear collider is shown to be an excellent environment for such high precision measurements of these SUSY parameters, which will provide an important probe of superparticles beyond reachable energies.
Date: January 1, 1998
Creator: Cheng, H.-C.
Partner: UNT Libraries Government Documents Department

Comments on the possible use of the SLC as an e{sup -}e{sup -} collider

Description: It has been suggested that the SLAC Linear Collider, which was designed to collide electrons with positrons, could be modified to collide electrons with electrons. A cursory examination of this suggestion leads us to conclude that such a modification is technically feasible, but by no means trivial. In this letter we outline the issues that must be addressed to realize this possibility.
Date: February 1, 1996
Creator: Erickson, R.
Partner: UNT Libraries Government Documents Department

Like-sign charged Higgs boson production in e{sup -}e{sup -} collisions at the NLC

Description: We consider the production of a pair of like-sign charged Higgs bosons in e{sup -} e{sup -} collisions at the NLC within the context of several electroweak models with extended symmetry breaking sectors. We find that the rate for this process, which proceeds through W{sup -}W{sup -} fusion, is a very sensitive probe of the nature of these extended Higgs sectors and that the corresponding cross sections can vary by as much as several orders of magnitude at NLC energies.
Date: October 1, 1995
Creator: Rizzo, T.G.
Partner: UNT Libraries Government Documents Department

Soft-X-ray spectra of highly charged Os, Bi, Th, and U ions in an electron beam ion trap

Description: Systematic variation of the electron-beam energy in an electron-beam ion trap has been employed to produce soft-X-ray spectra of Os, Bi, Th, and U with highest charge states ranging up to Ni-like ions. Guided by relativistic atomic structure calculations, the strongest lines have been identified with {Delta}n = 0 (n = 4 to n' = 4) transitions in Rb- to Cu-like ions. The rather weak 4p-4d transitions are much less affected by QED contributions than the dominant 4s-4p transitions. Our wavelength measurements consequently provide benchmarks with and (almost) without QED. Because the radiative corrections are not very sensitive to the number of electrons in the valence shell, our data, moreover, provide benchmarks for the evaluation of electron-electron interactions.
Date: December 3, 2004
Creator: Trabert, E.; Beiersdorfer, P.; Fournier, K. B. & Chen, M. H.
Partner: UNT Libraries Government Documents Department

Spin Transport in Semiconductor heterostructures

Description: The focus of the research performed under this grant has been the investigation of spin transport in magnetic semiconductor heterostructures. The interest in these systems is motivated both by their intriguing physical properties, as the physical embodiment of a spin-polarized Fermi liquid, as well as by their potential applications as spintronics devices. In our work we have analyzed several different problems that affect the spin dynamics in single and bi-layer spin-polarized two-dimensional (2D) systems. The topics of interests ranged from the fundamental aspects of the electron-electron interactions, to collective spin and charge density excitations and spin transport in the presence of the spin-orbit coupling. The common denominator of these subjects is the impact at the macroscopic scale of the spin-dependent electron-electron interaction, which plays a much more subtle role than in unpolarized electron systems. Our calculations of several measurable parameters, such as the excitation frequencies of magneto-plasma modes, the spin mass, and the spin transresistivity, propose realistic theoretical estimates of the opposite-spin many-body effects, in particular opposite-spin correlations, that can be directly connected with experimental measurements.
Date: February 22, 2011
Creator: Marinescu, Domnita Catalina
Partner: UNT Libraries Government Documents Department

The two-photon decay of 1s2s {sup 1}S{sup 0} states in heavy he-like atomic systems.

Description: In He-like systems the decay of the 1s2s {sup 1}S0 excited state to the 1s{sup 2} {sup 1}S0 ground state is not allowed. This excited state can only decay to the ground state via the emission of two photons. The spectral shape of the emitted continuum is determined by the complete structure of the atomic system as all bound and continuum P states contribute to the 2E1 decay. For very heavy atomic systems the {sup 3}P states ALSO have to be included and the normalized spectral shape changes with atomic number according to the relative strengths of both, the electron-electron interaction and of the relativistic effects. A brief survey on the variation of the spectral shape of the two-photon continuum with atomic number is given and compared to experiments ranging from He-like Ni to He-like Au with special emphasis on the heavy relativistic system. The data compare well with fully relativistic calculations.
Date: August 14, 2002
Creator: Mokler, P. H.; Dunford, R. W. & Kanter, E. P.
Partner: UNT Libraries Government Documents Department

Tests of QCD with polarized electrons

Description: We present three measurements that exploit the highly-polarized incident electrons of the SLC facility to probe QCD and the hadronization process. We observe preliminary evidence for leading particle production in hadronic decays of the Zo to light-quark pairs. In a high-purity sample of quark jets, the momentum spectra of p, A0, and K(-) are harder than those of p(bar), A(bar)0, and K(+), supporting the hypothesis that faster particles in jets are more likely to carry the primary quark or antiquark of the jet. Second, we present an improved limit on jet handedness, which seeks to measure the transport of quark spin through the hadronization process. Finally, we search for a correlation of the three jet event orientation with the Zo spin direction, which would indicate new physics beyond the Standard model.
Date: August 1, 1996
Creator: Pavel, T.J. & Collaboration, SLD
Partner: UNT Libraries Government Documents Department

Supersymmetry at linear colliders: The importance of being e{sup {minus}}e{sup {minus}}

Description: Advantages of the e{sup {minus}}e{sup {minus}} option at linear colliders for the study of supersymmetry are highlighted. The fermion number violating process e{sup {minus}}e{sup {minus}} {yields} {tilde e}{sup {minus}}{tilde e}{sup {minus}} provides unique opportunities for studies of slepton masses and flavor mixings. In particular, slepton mass measurements at the 100 MeV level through threshold scans of scalar pair production may be possible. Such measurements are over an order of magnitude better than those possible in e{sup +}e{sup {minus}} mode, require far less integrated luminosity, and may lead to precise, model-independent measurements of tan {beta}. Implications for studying gauginos and the importance of accurate beam polarimetry are also discussed.
Date: March 1, 1998
Creator: Feng, J.L.
Partner: UNT Libraries Government Documents Department

Many-body interactions in quasi-freestanding graphene

Description: The Landau-Fermi liquid picture for quasiparticles assumes that charge carriers are dressed by many-body interactions, forming one of the fundamental theories of solids. Whether this picture still holds for a semimetal such as graphene at the neutrality point, i.e., when the chemical potential coincides with the Dirac point energy, is one of the long-standing puzzles in this field. Here we present such a study in quasi-freestanding graphene by using high-resolution angle-resolved photoemission spectroscopy. We see the electron-electron and electron-phonon interactions go through substantial changes when the semimetallic regime is approached, including renormalizations due to strong electron-electron interactions with similarities to marginal Fermi liquid behavior. These findings set a new benchmark in our understanding of many-body physics in graphene and a variety of novel materials with Dirac fermions.
Date: June 3, 2011
Creator: Siegel, David; Park, Cheol-Hwan; Hwang, Choongyu; Deslippe, Jack; Fedorov, Alexei; Louie, Steven et al.
Partner: UNT Libraries Government Documents Department

The stochastic radiative transfer equation: quantum damping, Kirchoff's law and NLTE

Description: A method is presented based on the theory of quantum damping, for deriving a self consistent but approximate form of the quantum transport for photons interacting with fully ionized electron plasma. Specifically, we propose in this paper a technique of approximately including the effects of background plasma on a photon distribution function without directly solving any kinetic equations for the plasma itself. The result is a quantum Langevin equation for the photon number operator; the quantum radiative transfer equation. A dissipation term appears which is the imaginary part of the dielectric function for an electron gas with photon mediated electron-electron interactions due to absorption and re-emission. It depends only on the initial state of the plasma. A quantum noise operator also appears as a result of spontaneous emission of photons from the electron plasma. The thermal expectation value of this noise operator yields the emissivity which is exactly of the form of the Kirchoff-Planck relation. This non-zero thermal expectation value is a direct consequence of a fluctuation-dissipation relation (FDR).
Date: January 24, 2005
Creator: Graziani, F R
Partner: UNT Libraries Government Documents Department

Single-cycle nonlinear optics

Description: Nonlinear optics plays a central role in the advancement of optical science and laser-based technologies. We report on the confinement of the nonlinear interaction of light with matter to a single wave cycle and demonstrate its utility for time-resolved and strong-field science. The electric field of 3.3-femtosecond, 0.72-micron laser pulses with a controlled and measured waveform ionizes atoms near the crests of the central wave cycle, with ionization being virtually switched off outside this interval. Isolated sub-100-attosecond pulses of extreme ultraviolet light (photon energy {approx} 80 electron volts), containing {approx} 0.5 nanojoule of energy, emerge from the interaction with a conversion efficiency of {approx} 10{sup -6}. These tools enable the study of the precision control of electron motion with light fields and electron-electron interactions with a resolution approaching the atomic unit of time ({approx} 24 attoseconds).
Date: November 5, 2008
Creator: Quantenoptik, Max-Planck-Institut fur; Goulielmakis, E.; Schultze, M.; Hofstetter, M.; Yakovlev, V. S.; Gagnon, J. et al.
Partner: UNT Libraries Government Documents Department

Amplitude Modulation of Atomic Wavefunctions - Final Technical Report

Description: This project developed a novel wave function modulation technique. Other modulation techniques use tailored laser pulses to directly excite a time-dependent, modulated wave function from a ground state. Our technique began with one electron already excited to a time independent eigenfunction. Then, by using excitations of a second valence electron, we modulated the other wave function. Our technique had the benefit that it was very efficient, and required low power lasers with no need for precise phase or amplitude control. On the other hand it had the difficulty of being a multi-step laser excitation with a maximum repetition rate of 10 Hz. Over the course of this project, we showed that the technique did work, and work efficiently. However, it was easy to generalize. Since the modulation depended on a difference between electron-electron interactions with the inner electron in a ground or excited state, the efficiency of the modulation was strongly state dependent. For example, we never showed any significant modulation in our tests of barium states, while our strontium measurements did show efficient modulation as long as the state to be modulated was in the 5snd group with n between 30 and 50. We completed some studies of the dependence of the amplitude modulation as we varied the time between the excitation and de-excitation pulses applied to the inner electron. The amplitude of the nearest neighbor states was well described by Multi-Channel Quantum Defect theory, but farther satellites were problematical. This may have simply reflected the low density of measurement points, since the amplitudes of the farther satellites oscillate more quickly with time. As we developed our technique, we showed that we could directly measure autoionization decay rates in the time domain, and that the net effect of a state belonging to a Rydberg series was that exponential decay ...
Date: August 1, 2001
Creator: Cooke, William E.
Partner: UNT Libraries Government Documents Department

Beam-beam disruption and the case for a plasma lens in e{sup {minus}}e{sup {minus}} collisions

Description: In an e{sup {minus}}e{sup {minus}} collider, the mutual disruption of the beams will significantly decrease the luminosity, different from the case of e{sup +}e{sup {minus}} colliders, where the mutual attraction of the oppositely-charged beams leads to a self-focusing effect. In this paper the authors estimate the beam disruption at SLC and NLC energies using ABEL simulations. They then give estimates of how a plasma lens might improve the luminosity in e{sup {minus}}e{sup {minus}} collisions.
Date: March 1, 1996
Creator: Chen, P.; Spitkovsky, A. & Weidemann, A.W.
Partner: UNT Libraries Government Documents Department

Extended gauge models and e{sup -}e{sup -} {r_arrow} W{sup -}W{sup -} at the NLC

Description: We briefly discuss two possible manifestations of the lepton number violating reaction e{sup -}e{sup -} {r_arrow} W{sub i}{sup -}W{sub j}{sup -}, which probes the masses and mixings of heavy Majorana neutrinos, at the Next Linear Collider (NLC). Cross sections for this process are shown to be potentially quite large at center of mass energies of order 1-1.5 TeV.
Date: October 1, 1995
Creator: Rizzo, T.G.
Partner: UNT Libraries Government Documents Department

Theory of multi-electron recoil effects on x-ray lineshapes of metals

Description: Within the change of self-consistent field approximation, x-ray spectra can be considerably richer in many-electron phenomena than once suspected. With the finite number of electrons method, these spectra can be evaluated for realistic electron-hole interactions in free electron metals. Preliminary results indicate that metals with band structure can also be treated this way. However, theories of final-state interactions in metals await the reliable determinations of the screened potential of a core hole in a metal and realistic avaluation of the effects of electron-electron interactions. (GHT)
Date: January 1, 1980
Creator: Dow, J. D.; Swarts, C. A.; Bowen, M. A.; Mehreteab, E. & Satpathy, S. S.
Partner: UNT Libraries Government Documents Department

Emergence of the Persistent Spin Helix in Semiconductor Quantum Wells

Description: According to Noether's theorem, for every symmetry in nature there is a corresponding conservation law. For example, invariance with respect to spatial translation corresponds to conservation of momentum. In another well-known example, invariance with respect to rotation of the electron's spin, or SU(2) symmetry, leads to conservation of spin polarization. For electrons in a solid, this symmetry is ordinarily broken by spin-orbit (SO) coupling, allowing spin angular momentum to flow to orbital angular momentum. However, it has recently been predicted that SU(2) can be recovered in a two-dimensional electron gas (2DEG), despite the presence of SO coupling. The corresponding conserved quantities include the amplitude and phase of a helical spin density wave termed the 'persistent spin helix' (PSH). SU(2) is restored, in principle, when the strength of two dominant SO interactions, the Rashba ({alpha}) and linear Dresselhaus ({beta}{sub 1}), are equal. This symmetry is predicted to be robust against all forms of spin-independent scattering, including electron-electron interactions, but is broken by the cubic Dresselhaus term ({beta}{sub 3}) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as {alpha} {yields} {beta}{sub 1}. Here we observe experimentally the emergence of the PSH in GaAs quantum wells (QW's) by independently tuning {alpha} and {beta}{sub 1}. Using transient spin-grating spectroscopy (TSG), we find a spin-lifetime enhancement of two orders of magnitude near the symmetry point. Excellent quantitative agreement with theory across a wide range of sample parameters allows us to obtain an absolute measure of all relevant SO terms, identifying {beta}{sub 3} as the main SU(2) violating term in our samples. The tunable suppression of spin-relaxation demonstrated in this work is well-suited for application to spintronics.
Date: August 24, 2011
Creator: Koralek, Jake; Weber, Chris; Orenstein, Joe; Bernevig, Andrei; Zhang, Shoucheng; Mack, Shawn et al.
Partner: UNT Libraries Government Documents Department

Quantum Monte-Carlo Study of Electron Correlation in Heterostructure Quantum Dots

Description: The goal of this project is to study electron correlation in a confined geometry (quantum dots) within the two-dimensional quantum well in the sandwiches of two semiconductor materials. For these systems one is able to tune the electronic properties by controlling the size and the electron number, creating tremendous potential for novel applications. Much effort in this emerging field has been devoted to producing entangled states that are required for quantum information processing. At the same time, new physical phenomena have emerged from these artificial structures. Adding electrons to a quantum dot is more complicated than filling up discrete energy levels due to electron correlation. Therefore, our project is focusing on employing the state-of-the-art quantum Monte Carlo methods to study the electron-electron interaction. A close examination of the breakdown of Hund's rules and electron localization has been conducted in our simulations. The results are summarized in this report.
Date: November 12, 2006
Creator: Chou, Mei-Yin
Partner: UNT Libraries Government Documents Department

Resonances in high-energy colliding lepton beams

Description: The search for the correct higher symmetry of decay vertices is discussed. It is noted that something higher than SU(3) is certainly valid and colliding lepton beams can help find what it is. SU(6(/sub W/ and its limitations, L = 1, 2 mesons, and the two-photon processes e/sup +/ + e/sup -/ yields e/sup +/ + e/sup -/ + hadrons, and e/sup -/ + e/sup -/ yields e/sup -/ + e/sup -/ + hadrons are considered. (JFP)
Date: March 13, 1973
Creator: Rosner, J.L.
Partner: UNT Libraries Government Documents Department

Coulomb correlations and optical gap in polyacetylene

Description: A model including both electron-phonon coupling (as in the SSH Hamiltonian) and electron-electron interactions (on-site term U, nearest-neighbor term V) is treated within the variational scheme of Gutswiller. It is shown that for weak electron-phonon coupling the primary effect is a bond-order wave induced by electronic correlation, whereas the lattice dimerization is a secondary effect. Correspondingly the optical gap is mainly due to electronic correlation.
Date: January 1, 1986
Creator: Baeriswyl, D. & Maki, K.
Partner: UNT Libraries Government Documents Department

Signals from flavor changing scalar neutral currents at {mu}{sup +}{mu}{sup {minus}} colliders

Description: We illustrate the possibility of observing signals from Flavor Changing Neutral Currents, originating from the scalar sector of a Two Higgs Doublet Model. In particular, we focus on the tree level process {mu}{sup +}{mu}{sup {minus}} {yields} {bar t}c + {bar c}t, via scalar exchange in the s-channel, as a distinctive process for {mu}{sup +}{mu}{sup {minus}} colliders. 12 refs., 1 fig.
Date: January 19, 1996
Creator: Reina, L.
Partner: UNT Libraries Government Documents Department

Physics and technology of the next linear collider

Description: The authors present the prospects for the next generation of high-energy physics experiments with electron-positron colliding beams. This report summarizes the current status of the design and technological basis of a linear collider of center-of-mass energy 0.5--1.5 TeV, and the opportunities for high-energy physics experiments that this machine is expected to open. The physics goals discussed here are: Standard Model processes and simulation; top quark physics; Higgs boson searches and properties; supersymmetry; anomalous gauge boson couplings; strong WW scattering; new gauge bosons and exotic particles; e{sup {minus}}e{sup {minus}}, e{sup {minus}}{gamma}, and {gamma}{gamma} interactions; and precision tests of QCD.
Date: June 1, 1996
Partner: UNT Libraries Government Documents Department