Search Results (149 total)

The hallmark of nonlinear complexity phenomena in magnetohydrodynamic and plasma turbulence as well as all natural sciences is the appearance of intermittent fluctuating events. We introduce here a unique procedure that is both physically explicable and quantitatively accurate in deciphering the multifractal characteristics of intermittency. The generic character of the procedure provides a natural connection between the suggested spectrum based on rank order and the idea of one-parameter scaling for monofractals. We demonstrate the utility of this method using results obtained from a large scale two-dimensional magnetohydrodynamic simulation mimicking solar wind turbulence.

We report a high statistics measurement of Υ production with an 800  GeV/c proton beam on hydrogen and deuterium targets. The dominance of the gluon-gluon fusion process for Υ production at this energy implies that the cross section ratio, σ(p+d→Υ)/2σ(p+p→Υ), is sensitive to the gluon content in the neutron relative to that in the proton. Over the kinematic region 0<x_F<0.6, this ratio is found to be consistent with unity, in striking contrast to the behavior of the Drell-Yan cross section ratio σ(p+d)_DY/2σ(p+p)_DY. This result shows that the gluon distributions in the proton and neutron are very similar. The Υ production cross sections are also compared with the p+d and p+Cu cross sections from earlier measurements.

We report a measurement of the angular distributions of Drell-Yan dimuons produced using an 800  GeV/c proton beam on a deuterium target. The muon angular distributions in the dilepton rest frame have been measured over the kinematic range 4.5<m_μμ<15  GeV/c², 0<p_T<4  GeV/c, and 0<x_F<0.8. No significant cos⁡2ϕ dependence is found in these proton-induced Drell-Yan data, in contrast with the situation for pion-induced Drell-Yan data. The data are compared with expectations from models which attribute the cos⁡2ϕ distribution to a QCD vacuum effect or to the presence of the transverse-momentum-dependent Boer-Mulders structure function h₁^⊥. Constraints on the magnitude of the sea-quark h₁^⊥ structure functions are obtained.

We present a B-spline-based complex-rotation (BSCR) method with spin-dependent interaction for the study of atomic photoionization leading to multiple ionization channels dominated by doubly excited resonances for two-electron and divalent atoms. The degree of mixing between different spin states and between the bound and continuum components of the state function of the resonance state can be easily identified in the BSCR method. Its application to Mg photoionization gives good agreement with observed singlet-triplet mixed Mg spectra.

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5–11 and -t=2–7  GeV² with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0±0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

Based on a molecular mechanics concept, a nonlinear stick-spiral model is developed to investigate the mechanical behavior of single-walled carbon nanotubes (SWCNTs). The model is capable of predicting not only the initial elastic properties (e.g., Young’s modulus) but also the stress-strain relations of a SWCNT under axial, radial, and torsion conditions. The elastic properties, ultimate stress, and failure strain under various loading conditions are discussed and special attention has been paid to the effects of the tube chirality and tube size. Some unique mechanical behaviors of chiral SWCNTs, such as axial strain-induced torsion, circumferential strain-induced torsion, and shear strain-induced extension are also studied. The predicted results from the present model are in good agreement with existing data, but very little computational cost is needed to yield them.

The rise and fall time behavior of a pulsed microwave oscillator is a problem of academic interest. It is also of importance to radar and other applications because it can lead to phase and frequency jitters or even lock the entire pulse into an undesired mode. Here we present a study of the rise and fall time behavior in the gyrotron backward-wave oscillator (gyro-BWO). Single-mode simulations reveal that, during the rise and fall portions of the electron beam pulse, oscillation frequencies of the axial modes vary in such a way that their transit angles remain at the respective optimum values. Thus, axial mode competition and mode switching can readily take place in these transient stages. Time-dependent simulations demonstrate that, under both the gradual and instant turn-on conditions, the axial modes compete in a pattern governed by the characteristic asymmetry of the mode profiles. Other aspects of physics interest include the analysis and explanation of a resultant hysteresis effect between the rise and fall portions of the beam pulse. These understandings are expected to provide the basis for achieving a stable gyro-BWO operating at a single mode throughout the entire beam pulse.

The gyrotron traveling-wave amplifier employing the distributed-loss scheme is capable of very high gain and effective in suppressing the global absolute instabilities. This study systematically characterizes the local absolute instabilities and their transitional behavior. The local absolute instabilities are analyzed using a model that incorporates the penetration of the field from the copper section into the lossy section. The axial modes were characterized from the perspective of beam-wave interaction and were found to share many characteristics with the global modes. The transition from global modes to local modes as the distributed loss increases was demonstrated. The electron transit angle in the copper section, which determines the feedback criterion, governs the survivability of an oscillation. In addition, the oscillation thresholds predicted using this model are more accurate than those obtained using a simplified model.

In this Reply, we reexamine the beating Shubnikov–de Haas oscillations by a nonlinear curve-fitting technique. The results do not support the arguments of Tang [Phys. Rev. B 73, 037301 (2006)], and it is unlikely that the beating Shubnikov–de Haas oscillations we observed in Al_xGa_1−xN∕GaN heterostructures originate from magnetointersubband scattering.

The axial modes of the gyrotron backward-wave oscillator (gyro-BWO) each exhibit a distinctive asymmetry in axial field profile. As a result, and in sharp contrast to the behavior of the familiar resonator-based gyrotron oscillator, particle simulations of the gyro-BWO reveal a radically different pattern of mode competition in which a fast-growing and well-established mode is subsequently suppressed by a later-starting mode with a more favorable field profile. This is verified in a Ka-band experiment and the interaction dynamics are elucidated with a time-frequency analysis.

Based on a molecular mechanics model, analytical solutions are obtained for the critical buckling strain of multiwalled carbon nanotubes (MWNT’s) under axial compression and bending. We show that only part of the outer layers buckles first while the remaining inner part remains stable in a very thick MWNT, which is quite different from the initial buckling mode of a relatively thin MWNT in which all individual tubes buckle simultaneously. Such a difference in the initial buckling modes results in quite different size effects on the critical buckling strain of thin and thick MWNT’s. For instance, inserting more inner individual tubes may increase the critical buckling strain of a thin MWNT, but cannot increase the critical buckling strain of a thick tube. The effects of tube size on the initial buckling wavelength are also examined, and it is shown that the initial buckling wavelength is weakly dependent on the thickness of the MWNT.

Compton scattering from the proton was investigated at s=6.9  GeV² and t=-4.0  GeV² via polarization transfer from circularly polarized incident photons. The longitudinal and transverse components of the recoil proton polarization were measured. The results are in disagreement with a prediction of perturbative QCD based on a two-gluon exchange mechanism, but agree well with a prediction based on a reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton.

The differential cross sections for the γn→π^-p and the γp→π⁺n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50^° to 110^°. The π^- and π⁺ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing π⁺ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios [dσ/dt(γn→π^-p)/dσ/dt(γp→π⁺n)] at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.

This paper examines the physical mechanisms of reading out spatial-spectral absorption features in an inhomogeneously broadened medium using linear frequency-chirped electric fields. A Maxwell-Bloch model using numerical calculation for angled beams with arbitrary phase modulation is used to simulate the chirped field readout process. The simulation results indicate that any spatial-spectral absorption feature can be read out with a chirped field with the appropriate bandwidth, duration, and intensity. Mapping spectral absorption features into temporal intensity modulations depends on the chirp rate of the field. However, when probing a spatial-spectral grating with a chirped field, a beat signal representing the grating period can be created by interfering the emitted photon echo chirped field with a reference chirped field, regardless of the chirp rate. Comparisons are made between collinear and angled readout configurations. Readout signal strength and spurious signal distortions are investigated as functions of the grating strength and the Rabi frequency of the readout pulse. Using a collinear readout geometry, distortions from optical nutation on the transmitted field and higher-order harmonics are observed, both of which are avoided in an angled beam geometry.

The absolute instability is a subject of considerable physics interest as well as a major source of self-oscillations in the gyrotron traveling-wave amplifier (gyro-TWT). We present a theoretical study of the absolute instabilities in a TE₀₁ mode, fundamental cyclotron harmonic gyro-TWT with distributed wall losses. In this high-order-mode circuit, absolute instabilities arise in a variety of ways, including overdrive of the operating mode, fundamental cyclotron harmonic interactions with lower-order modes, and second cyclotron harmonic interaction with a higher-order mode. The distributed losses, on the other hand, provide an effective means for their stabilization. The combined configuration thus allows a rich display of absolute instability behavior together with the demonstration of its control. We begin with a study of the field profiles of absolute instabilities, which exhibit a range of characteristics depending in large measure upon the sign and magnitude of the synchronous value of the propagation constant. These profiles in turn explain the sensitivity of oscillation thresholds to the beam and circuit parameters. A general recipe for oscillation stabilization has resulted from these studies and its significance to the current TE₀₁-mode, 94-GHz gyro-TWT experiment at UC Davis is discussed.

We present measurements of the polarization of the J/ψ produced in 800-GeV proton interactions with a copper target. Polarization of the J/ψ is sensitive to the cc̅ production and hadronization processes. A longitudinal polarization is observed at large x_F, while at small x_F the state is produced essentially unpolarized or slightly transversely polarized. No significant variation of the polarization is observed versus p_T.

We have measured the nuclear transparency of the fundamental process γn→π^-p in ⁴He. These measurements were performed at Jefferson Lab in the photon energy range of 1.6–4.5 GeV and at θ_cm^π=70° and 90°. These measurements are the first of their kind in the study of nuclear transparency in photoreactions. They also provide a benchmark test of Glauber calculations based on traditional models of nuclear physics. The transparency results suggest deviations from the traditional nuclear physics picture. The momentum transfer dependence of the measured nuclear transparency is consistent with Glauber calculations that include the quantum chromodynamics phenomenon of color transparency.

We have measured the differential cross section for the γn→π^-p and γp→π⁺n reactions at θ_c.m.=90° in the photon energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at E_γ≳3.3   GeV exhibit a global scaling behavior for both π^- and π⁺ photoproduction, consistent with the constituent counting rule and the existing π⁺ photoproduction data. Possible oscillations around the scaling value are suggested by these new data. The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio of exclusive π^- to π⁺ photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.

The reaction p(e,e^′π⁺)X⁰ was studied with two high-resolution magnetic spectrometers to search for narrow baryon resonances. A missing mass resolution of 2.0 MeV was achieved. A search for structures in the mass region of 0.97<M_X⁰<1.06 GeV yielded no significant signal. The yield ratio of p(e,e^′π⁺)X⁰/p(e,e^′π⁺)n was determined to be (-0.35±0.35)×10^-3 at 1.004 GeV and (0.34±0.42)×10^-3 at 1.044 GeV. This measurement clearly demonstrated the potential of high-resolution missing mass searches in coincidence experiments.

Formation of axial modes in the gyrotron backward-wave oscillator is examined in the perspective of optimum conditions for beam-wave interactions. Distinctive linear properties are revealed and interpreted physically. Nonlinear implications of these properties (specifically, the role of high-order axial modes) are investigated with time-dependent simulations. Nonstationary oscillations exhibit self-modulation behavior while displaying no evidence of axial mode competition. Reasons for the erratic frequency tuning are investigated and stable tuning regimes are identified as a remedy.

The first complete measurements of the angular distributions of the two-body deuteron photodisintegration differential cross section at photon energies above 1.6 GeV were performed at the Thomas Jefferson National Accelerator Facility. The results show a persistent forward-backward asymmetry up to E_γ=2.4 GeV, the highest-energy measured in this experiment. The Hard Rescattering and the Quark-Gluon string models are in fair agreement with the results.

We present measurements of the recoil proton polarization for the ¹H(γ→,p→)π⁰ reaction for θ_c.m.^π=60°–135° and for photon energies up to 4.1 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. Various theoretical models are compared with the results. No evidence for hadron helicity conservation is observed. Models that employ factorization are not favored. It appears from the strong angular dependence of the induced polarization at photon energies of 2.5 and 3.1 GeV that a relatively high spin resonance or background amplitude might exist in this energy region.

We have studied the electronic properties of Al_xGa_1-xN/GaN heterostructures by using Shubnikov–de Haas (SdH) measurement. Two SdH oscillations were detected on the samples of x=0.35 and 0.31, due to the population of the first two subbands with the energy separations of 128 and 109 meV, respectively. For the sample of x=0.25, two SdH oscillations beat each other, probably due to a finite zero-field spin splitting. The spin-splitting energy is equal to 9.0 meV. The samples also showed a persistent photoconductivity effect after illuminating by blue light-emitting diode.

Intense ultranarrow resonances have been observed in the photoionization of Mg from the singly excited 3smd ¹D (m=3,4,5,6) states to the doubly excited 3png ¹F (n=5,6,7) autoionization states using stepwise laser excitation and time-of-flight mass spectrometry detection. The reported experimental energy resolution at an order of 10^-7 of the photon energy represents a considerable improvement over the best photon energy resolution of about one part in 10⁵ for the synchrotron radiation light sources. The ultra-narrow 3png ¹F resonances acquire a significant strength for the normally weak 3smd ¹D→3png ¹F two-electron transition due to a strong configuration interaction with an overlapping broad 3pnd ¹F autoionization series and the 3sεf ¹F continuum. The measured line positions and transition widths are in close agreement with the theoretical results derived from a B-spline-based configuration-interaction calculation.

The ratio of the electric and magnetic form factors of the proton G_Ep/G_Mp, which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e→p→ep→ reaction. The new data presented span the range 3.5<Q²<5.6 GeV² and are well described by a linear Q² fit. Also, the ratio sqrt[Q²] F_2p/F_1p reaches a constant value above Q²  =  2 GeV².