Search Results (27 total)

Jet cross sections were measured in charged-current deep inelastic e^±p scattering at high boson virtualities Q² with the ZEUS detector at HERA II using an integrated luminosity of 0.36  fb^-1. Differential cross sections are presented for inclusive-jet production as functions of Q², Bjorken x and the jet transverse energy and pseudorapidity. The dijet invariant mass cross section is also presented. Observation of three- and four-jet events in charged-current e^±p processes is reported for the first time. The predictions of next-to-leading-order (NLO) QCD calculations are compared to the measurements. The measured inclusive-jet cross sections are well described in shape and normalization by the NLO predictions. The data have the potential to constrain the u and d valence-quark distributions in the proton if included as input to global fits.

We measure the microwave signals produced by spin-torque-driven magnetization dynamics excited by direct currents in patterned magnetic multilayer devices at room temperature as a function of the angle of a magnetic field applied in the sample plane. We find strong variations in the frequency linewidth of the signals, with a decrease by more than a factor of 20 as the field is rotated from the magnetic easy axis to the in-plane hard axis. Based on micromagnetic simulations, we identify these variations as due to a transition from spatially incoherent to coherent precession.

The torque generated by the transfer of spin angular momentum from a spin-polarized current to a nanoscale ferromagnet can switch the orientation of the nanomagnet much more efficiently than a current-generated magnetic field and is therefore in development for use in next-generation magnetic random access memory (MRAM). Up to now, experiments have focused on spin-torque switching driven by simple square-wave current pulses. Here we present measurements showing that spin transfer from a microwave-frequency current pulse can produce a resonant excitation of a nanomagnet and improved switching characteristics in combination with a square current pulse. With the assistance of a microwave-frequency pulse, the switching time is reduced and achieves a narrower distribution than when driven by a square current pulse alone, and this can permit significant reductions in the integrated power required for switching. Resonantly excited switching may also enable alternative, more compact MRAM circuit architectures.

A symmetrical E-shaped metamaterial is investigated in this paper. Numerical simulations disclose the two electromagnetic resonances and the superwide negative-refractive band of this structure. The distributions of the induced current in the E-shaped copper wires show that these two electromagnetic resonances originate from the current flowing in the different C-shaped rings. The retrieved negative-refractive band is superwide, and the negative refraction with high transmission occurs in the different bands. The metamaterial with two or even more electromagnetic resonances offers an opportunity for the realization of wide-band negative refraction.

Flat directions are a generic feature of the scalar potential in supersymmetric gauge field theories. They can arise, for example, from D-terms associated with an extra Abelian gauge symmetry. Even when supersymmetry is broken softly, there often remain directions in the scalar field space along which the potential is almost flat. Upon breaking a gauge symmetry along one of these almost-flat directions, cosmic strings may form. Relative to the standard cosmic string picture based on the Abelian Higgs model, these flat-direction cosmic strings have the extreme type-I properties of a thin gauge core surrounded by a much wider scalar field profile. We perform a comprehensive study of the microscopic, macroscopic, and observational characteristics of this class of strings. We find many differences from the standard string scenario, including stable higher winding-mode strings, the dynamical formation of higher mode strings from lower ones, and a resultant multitension scaling string network in the early universe. These strings are only moderately constrained by current observations, and their gravitational wave signatures may be detectable at future gravity wave detectors. Furthermore, there is the interesting but speculative prospect that the decays of cosmic string loops in the early universe could be a source of ultrahigh-energy cosmic rays or nonthermal dark matter. We also compare the observational signatures of flat-direction cosmic strings with those of ordinary cosmic strings as well as (p,q) cosmic strings motivated by superstring theory.

The cross section for high-E_T dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8  pb^-1. The events were required to have a virtuality of the incoming photon, Q², of less than 1  GeV² and a photon-proton center-of-mass energy in the range 142<W_γp<293  GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of E_T^jet1>20  GeV and E_T^jet2>15  GeV and pseudorapidity (with respect to the proton beam direction) requirements of -1<η^jet1,2<3, with at least one of the jets satisfying -1<η^jet<2.5. The measurements show sensitivity to the parton distributions in the photon and proton and to effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.

The current-voltage (I-V) characteristics of various MgB₂ films have been studied at different magnetic fields parallel to the c axis. At fields μ₀H between 0 and 5  T, vortex liquid-glass transitions were found in the I-V isotherms. Consistently, the I-V curves measured at different temperatures show a scaling behavior in the framework of quasi-two-dimension (quasi-2D) vortex-glass theory. However, at μ₀H≥5  T, a finite dissipation was observed down to the lowest temperature here, T=1.7  K, and the I-V isotherms did not scale in terms of any known scaling law, of any dimensionality. We suggest that this may be caused by a mixture of σ band vortices and π band quasiparticles. Interestingly, the I-V curves at zero magnetic field can still be scaled according to the quasi-2D vortex-glass formalism, indicating an equivalent effect of self-field due to persistent current and applied magnetic field.

The collisions of the isocharged sequence ions of q=6 (C⁶⁺, N⁶⁺, O⁶⁺, F⁶⁺, Ne⁶⁺, Ar⁶⁺, and Ca⁶⁺), q=7 (F⁷⁺, Ne⁷⁺, S⁷⁺, Ar⁷⁺, and Ca⁷⁺), q=8 (F⁸⁺, Ne⁸⁺, Ar⁸⁺, and Ca⁸⁺), q=9 (F⁹⁺, Ne⁹⁺, Si⁹⁺, S⁹⁺, Ar⁹⁺, and Ca⁹⁺) and q=11 (Si¹¹⁺, Ar¹¹⁺, and Ca¹¹⁺) with helium at the same velocities were investigated. The cross-section ratios of the double-electron transfer (DET) to the single-electron capture (SEC) σ^DET∕σ^SEC and the true double-electron capture (TDC) to the double-electron transfer σ^TDC∕σ^DET were measured. It shows that for different ions in an isocharged sequence, the experimental cross-section ratio σ^DET∕σ^SEC varies by a factor of 3. The results confirm that the projectile core is another dominant factor besides the charge state and the collision velocity in slow (0.35−0.49v₀; v₀ denotes the Bohr velocity) highly charged ions (HCIs) with helium collisions. The experimental cross-section ratio σ^DET∕σ^SEC is compared with the extended classical over-barrier model (ECBM) [A. Bárány , Nucl. Instrum. Methods Phys. Res. B 9, 397 (1985)], the molecular Coulombic barrier model (MCBM) [A. Niehaus, J. Phys. B 19, 2925 (1986)], and the semiempirical scaling laws (SSL) [N. Selberg , Phys. Rev. A 54, 4127 (1996)]. It also shows that the projectile core properties affect the initial capture probabilities as well as the subsequent relaxation of the projectiles. The experimental cross-section ratio σ^TDC∕σ^DET for those lower isocharged sequences is dramatically affected by the projectile core structure, while for those sufficiently highly isocharged sequences, the autoionization always dominates, hence the cross-section ratio σ^TDC∕σ^DET is always small.

A large data set of charged-pion (π^±) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms.

Matter fields in the minimal supersymmetric standard model are chiral supermultiplets in fundamental (or singlet) representations of the standard model gauge group. In this paper we introduce chiral superfields in the adjoint representation of SU(3)_C and study the effective field theory and phenomenology of them. These states are well motivated by intersecting D-brane models in which additional massless adjoint chiral supermultiplets appear generically in the low-energy spectrum. Although it has been pointed out that the existence of these additional fields may make it difficult to obtain asymptotic freedom, we demonstrate that this consideration does not rule out the existence of adjoints. The QCD gauge coupling can be perturbative up to a sufficiently high scale, and therefore a perturbative description for a D-brane model is valid. The full supersymmetric and soft SUSY breaking Lagrangians and the resulting renormalization group equations are given. Phenomenological aspects of the adjoint matter are also studied, including the decay and production processes. The similarity in gauge interaction between the adjoint fermion and gluino facilitates our study on these aspects. It is found that these adjoint multiplets can give detectable signals at colliders and satisfy the constraints from cosmology.

Studies of the dynamics of longitudinal space-charge waves in space-charge dominated beams propagating through a transport channel with a long solenoid are performed at the University of Maryland. In this paper, we report some experimental results on the energy modulations converted from density modulations. By changing the working conditions of the electron gun, pure initial density modulations are generated. Energy perturbation waveforms are measured with a high-resolution energy analyzer. The experimental results are compared with both the linear theory and the simulation results. Good agreements are achieved for the relationship between the energy and current perturbation strengths.

Using the color-magnetic interaction Hamiltonian with SU(3) flavor symmetry breaking, we perform a schematic study of the masses of the J^P=1⁺ tetraquarks in the antidecuplet representation. After diagonalizing the mass matrix, we find the uds̅ s̅ tetraquark could lie as low as about 1350 MeV. It decays into K⁺K⁰π⁰, K⁺K⁺π^-, K⁰K⁰π⁺ via P-wave. The dual suppression from the not-so-big three-body phase space and P-wave decay barrier may render this exotic state rather narrow. Future experimental exclusion of this state will cast doubt on the validity of applying the simple color-magnetic Hamiltonian to the multiquark system.

The low-frequency (10²–10⁵ Hz) dielectric properties and dc resistivity of oxygen-doped La₂CuO_4+y ceramic samples were investigated as a function of temperature (23–300 K). The as-prepared sample featuring phase separation shows very high dielectric constant ε^′>10⁵ above 150 K, which was found to originate from the hopping motion of localized holes. Below 150 K, the relaxation process due to the condensation of the holes can be observed in the dielectric spectra. When the sample was annealed at different temperatures in reduced atmosphere to tune the oxygen content, the phase separation disappears accompanied by the absence of the relaxation in the annealed sample. Our results give strong support to the phase separation model and evidence the inhomogeneous distribution of holes in the oxygen-doped La₂CuO_4+y ceramic samples investigated.

We report a new experimental study of the growth of longitudinal energy spread in a space-charge-dominated electron beam, with a beam energy of several keV and beam current of approximately 100 mA. At relatively low beam densities, we measure growing energy spreads with distance along the transport channel, which are in remarkably good agreement with the theory of energy relaxation via Coulomb collisions. At higher beam densities, however, anomalous energy spreads exceeding the predictions of the relaxation theory are observed, which, we believe, could be caused by collective longitudinal-transverse instabilities observed in computer simulation studies. The onset of these instabilities occurs after several plasma periods according to calculations.

We investigated the effect of alloying on the upper critical field H_c2 for 12 MgB₂ films, in which disorder was introduced by growth, carbon doping or He-ion irradiation, finding a significant H_c2 enhancement in C-alloyed films, and an anomalous upward curvature of H_c2(T). Record high values of H_c2^⊥(4.2)≈35 T and H_c2^‖(4.2)≈51 T were observed perpendicular and parallel to the ab plane, respectively. The temperature dependence of H_c2(T) is described well by a theory of dirty two-gap superconductivity. Extrapolation of the experimental data to T=0 suggests that H_c2^‖(0) may approach the paramagnetic limit of ∼70 T.

Characterization of beam energy spread in a space-charge dominated beam is very important to understanding the physics of intense beams. It is believed that coupling between the transverse and longitudinal directions via Coulomb collisions will cause an increase of the beam longitudinal energy spread. At the University of Maryland, experiments have been carried out to study the energy evolution in such intense beams with a high-resolution retarding field energy analyzer. The temporal beam energy profile along the beam pulse has been characterized at the distance of 25 cm from the anode of a gridded thermionic electron gun. The mean energy of the pulsed beams including the head and tail is reported here. The measured rms energy spread is in good agreement with the predictions of the intrabeam scattering theory. As an application of the beam energy measurement, the input impedance between the cathode and the grid due to beam loading can be calculated and the impedance number is found to be a constant in the operation region of the gun.

We calculated the electronic structures of the Heusler alloy Ni₂FeGa for both the cubic and the orthorhombic structures by self-consistent full-potential linearized-augmented plane-wave method. The localized moment of Fe atom is interpreted based on the electronic structure and the popular explanation of the localized moment of Mn in Heusler alloy X₂MnY. Comparing the density of states of cubic and orthorhombic structures, we observed that a Ni peak near the density of states of d band for the cubic structure splits for the orthorhombic structure, indicating a band Jahn-Teller mechanism should be responsible for the structural transition. Accompanied by this transformation, an increase of Ni moment and magnetization redistribution occurred. Temperature-dependence anisotropy field shows an evidence of martensitic transformation between 125 and 190 K. The magnetic behavior seems to contain a transition from Heisenberg-like at temperature below 70 K to itinerant magnetism at temperature higher than 160 K upon martensitic transformation. Temperature dependence of saturation magnetization reveals the spontaneous magnetization at martensite and parent phase are 3.170μ_B and 3.035μ_B, respectively. The calculated magnetic moment at martensite is 3.171μ_B, which is quite consistent with the experimental value. The magnetic moment of Fe and Ni atom in Heusler alloy Ni₂FeGa is analyzed based on the computational results and the experimental magnetization curves. It is found that the magnetic moment of Fe atoms is about 10–43% larger than that of α-Fe.

Theoretical and experimental work has been carried out to study the longitudinal space-charge effects in a retarding field energy analyzer. A one-dimensional, steady state model for both a monoenergetic beam and a thermal beam has been developed for this purpose. Potential improvements of using two-dimensional and time-dependent solutions are also briefly discussed. The study shows that, if the current density inside the device is higher than a critical value, the longitudinal space-charge effect and the formation of a potential minimum similar to the virtual cathode formation in an electron gun will distort the measured energy spectrum. The measured FWHM and the rms energy spread will be affected. The measured mean energy will also be shifted toward the low-energy side. By using a two-dimensional correction, the theoretical model also qualitatively explains the appearance of a visible tail at the high-energy side of the spectrum, as observed in experiments. According to the theory, to avoid this measurement distortion due to the longitudinal space charge, care has to be taken to limit the current density inside the device.

Evolution of spirals during molecular beam epitaxy growth of GaN films on 6H-SiC(0001) was studied by in situ scanning tunneling microscopy. It was found that dislocations emerge at the film surface, creating straight steps with orientation along 〈11¯00〉 directions with a density of 10¹⁰ cm^-2 for 40-nm-thick films. During subsequent growth, these straight steps wind around dislocations and develop into spirals with a density of 10⁹ cm^-2 for 100-nm-thick films. The spirals can be classified into three types: single arm, interlocking double arm, and closed loop. The first two types originate from steps with one end pinned, and the third type results from steps with both ends pinned. At film thickness larger than 200 nm, these spirals further evolve into spiral mounds with a density of 10⁷ cm^-2. Based on the Burton, Cabrera, and Frank theory, a model is proposed to explain the formation of different types of spirals and the reduction of their densities.

Eight beams of 0.35-μm laser with pulse duration of about 1.0 ns and energy of 260 J per beam were injected into a cylindrical cavity to generate intense x-ray radiation on the Shengguang II high power laser facility. Plastic foils with a thickness in the range of about 3.0–45 μm were attached on the diagnostic hole of the cavity and ablated by the intense x-ray radiation. The radiative energy transport through plastic foils with different thicknesses has been studied experimentally. The burn-through time of the plastic foils has been obtained. For comparison, we also simulated the experimental results with Planckian and non-Planckian x-ray spectrum source, respectively. It is shown that for thick plastic foil the simulation with non-Planckian x-ray spectrum source is in good agreement with the experiment.

We have developed a compact high-resolution retarding field energy analyzer for measuring the energy spread of space-charge-dominated electron beams. This energy analyzer has a cylindrical electrode to overcome the defocusing effects due to space-charge forces, beam trajectories, aperture effect, etc. The device provides excellent spatial and temporal information on the beam energy spread. Single-particle simulation shows that this energy analyzer has very good resolution for low-energy electron beams of several kilovolts and with large divergence angles. The energy analyzer has been tested with 2.5 keV, 60 mA electron beams. The measured energy spread is also compared with the theoretical calculations taking into account two main energy spread sources, namely, the Boersch effect and the longitudinal-longitudinal relaxation.

The neutral nets in the lattice models of proteins are composed of a group of similar sequences that encode for the same native structure. The maximal neutral nets in an HP and an AB lattice model are investigated by exhaustive enumeration in the conformation space. The result demonstrates that the performance of mis-specified potential functions can be improved significantly by averaging over sequences in the neutral nets.

The formation and decomposition of C₆₁ clusters by collisions of carbon atoms with C₆₀ clusters have been studied within tight-binding molecular-dynamics simulations. It is found that a carbon projectile can be strongly bound outside a C₆₀ cage; C₆₁ may decay into C₆₀+C, in which the projectile carbon atom substitutes for a carbon atom in the C₆₀ target, into C₅₉+C₂ and C₅₈+C+C₂. The results are in good agreement with experiments. The C@C₆₀ clusters are found to be formed at high collision energies. The critical energies for formations and decompositions of C₆₁ clusters with the projectile carbon atoms incident in radial directions are given.