Search Results (104 total)

Relating to magnified image formation systems, we consider two processes in this work: quantum and classical incoherent imaging. There are similarities and differences between them. A similarity is that the optical transfer functions (OTFs) of both systems are obtained by calculating a correlation function. Concerning the differences, only two were found. The first difference is that, unlike classical incoherent imaging, quantum imaging with photon pairs generated by spontaneous parametric down-conversion is sensitive to the phase in the function which describes the object. The second difference is that the OTF of the quantum imaging system is a cross-correlation function of the OTFs of the signal and idler arms, while the OTF of the classical incoherent imaging system is an autocorrelation of the only pupil function existing in this case. We explore experimentally the second difference. We report the observation of a highly resolved image produced by a quantum light source in a setup where an incoherent light source is unable to spatially resolve the image of the object. The spatial resolution of the image is studied in a situation where the size of the object is magnified at the image.

Lipid rafts, defined as domains rich in cholesterol and sphingolipids, are involved in many important plasma membrane functions. Recent studies suggest that the way cells handle membrane cholesterol is fundamental in the formation of such lateral heterogeneities. We propose to model the plasma membrane as a nonequilibrium phase-separating system where cholesterol is dynamically incorporated and released. The model shows how cellular regulation of membrane cholesterol may determine the nanoscale lipid organization when the lipid mixture is close to a phase separation boundary, providing a plausible mechanism for raft formation in vivo.

The present experiment exploits the interference between the deeply virtual Compton scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D(e→,e^′γ)X cross section measured at Q²=1.9  GeV² and x_B=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to E_q, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.

We have developed an adapted model in order to describe the ferromagnetic resonance (FMR) spectra in a trilayer system in which two continuous ferromagnetic films are coupled by a granular magnetic spacer. The model allowed us to study the influence that different parameters (e.g., the Fe volume concentration and the thickness of the granular spacer, the exchange coupling field between layers, the microwave frequency, etc.) have on the overall line shape of the spectra. We present the general results predicted by the model and compare them with FMR experimental measurements made on a particular trilayer {Fe∕[Fe(x)-SiO₂(1−x)](t)∕Ni₈₀Fe₂₀} formed by two continuous ferromagnetic layers, Fe and Permalloy (Ni₈₀Fe₂₀), separated by a granular film of Fe-SiO₂, in which we changed the Fe volume concentration x (0.45<x<0.85), and the thickness t (t=1,2,4,9, and 18  nm) of the granular spacer. Room-temperature FMR measurements were made at the Q (ν=34  GHz) and X bands (ν=9.5  GHz) with the external field applied parallel to the film plane. Two well-resolved absorption modes, one at low fields and another at higher fields, were generally observed. From the dependence of the resonance field and the relative intensity of these modes on x and t it was possible to deduce that the granular layer strongly interacts with the Fe layer, whereas the Permalloy layer is only weakly coupled with the rest of the layers.

We investigated simultaneously the ¹²C(e,e^′p) and ¹²C(e,e^′pp) reactions at Q²=2  (GeV/c)², x_B=1.2, and in an (e, e^′p) missing-momentum range from 300 to 600  MeV/c. At these kinematics, with a missing momentum greater than the Fermi momentum of nucleons in a nucleus and far from the delta excitation, short-range nucleon-nucleon correlations are predicted to dominate the reaction. For (9.5±2)% of the ¹²C(e,e^′p) events, a recoiling partner proton was observed back-to-back to the ¹²C(e,e^′p) missing-momentum vector, an experimental signature of correlations.

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a ¹²C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed _Λ¹²B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Λ peak and the 11 MeV p-wave Λ peak.

A high-resolution (σ_instr.=1.5 MeV) search for narrow states (Γ<10 MeV) with masses of M_x≈1500–1850 MeV in ep→e^′K⁺X,e^′K^-X, and e^′π⁺X electroproduction at small angles and low Q² was performed. These states would be candidate partner states of the reported Θ⁺(1540) pentaquark. No statistically significant signal was observed in any of the channels at 90% C.L. Upper limits on forward production were determined to be between 0.8% and 4.9% of the Λ(1520) production cross section, depending on the channel and the assumed mass and width of the state.

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.

We report on a study of the longitudinal to transverse cross section ratio, R=σ_L/σ_T, at low values of x and Q², as determined from inclusive inelastic electron-hydrogen and electron-deuterium scattering data from Jefferson Laboratory Hall C spanning the four-momentum transfer range 0.06<Q²<2.8  GeV². Even at the lowest values of Q², R remains nearly constant and does not disappear with decreasing Q², as might be expected. We find a nearly identical behavior for hydrogen and deuterium.

We have examined the spin structure of the proton in the region of the nucleon resonances (1.085  GeV<W<1.910  GeV) at an average four momentum transfer of Q²=1.3  GeV². Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries A_∥ and A_⊥ to high precision, and extracted the asymmetries A₁ and A₂, and the spin structure functions g₁ and g₂. We found a notably nonzero A_⊥, significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.

We have measured the transverse asymmetry A_T^' in the quasielastic ³He→(e→,e^') process with high precision at Q² values from 0.1 to 0.6  (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted at Q² values of 0.1 and 0.2  (GeV/c)² using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of ³He→(e→,e^'). We also extracted the neutron magnetic form factor G_Mⁿ at Q² values of 0.3 to 0.6  (GeV/c)² based on plane wave impulse approximation calculations.

The existence of chaotic behavior for the geodesics of the test particles orbiting compact objects is a subject of much current research. Some years ago, Guéron and Letelier [Phys. Rev. E 66, 046611 (2002)] reported the existence of chaotic behavior for the geodesics of the test particles orbiting compact objects like black holes induced by specific values of the quadrupolar deformation of the source using as models the Erez--Rosen solution and the Kerr black hole deformed by an internal multipole term. In this work, we are interested in the study of the dynamic behavior of geodesics around astrophysical objects with intrinsic quadrupolar deformation or nonisotropic stresses, which induces nonvanishing quadrupolar deformation for the nonrotating limit. For our purpose, we use the Tomimatsu-Sato spacetime [Phys. Rev. Lett. 29 1344 (1972)] and its arbitrary deformed generalization obtained as the particular vacuum case of the five parametric solution of Manko et  al. [Phys. Rev. D 62, 044048 (2000)] characterizing the geodesic dynamics throughout the Poincaré sections method. We found only regular motion for the geodesics in the Tomimatsu-Sato δ=2 solution. Additionally, using the deformed generalization of Tomimatsu-Sato δ=2 solution given by Manko et al. we found chaotic motion for oblate deformation instead of prolate deformation, which is in contrast to the results by Guéron and Letelier. It opens the possibility that the particles forming the accretion disk around a large variety of different astrophysical bodies (nonprolate, e.g., neutron stars) could exhibit chaotic dynamics. We also conjecture that the existence of an arbitrary deformation parameter is necessary for the existence of chaotic dynamics.

We present the first measurements of the e→p→epγ cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q² dependence (from 1.5 to 2.3  GeV²) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q². The helicity-independent cross section is also measured at Q²=2.3  GeV². We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.

The ratio of the proton's electric to magnetic form factor, G_E/G_M, can be extracted in elastic electron-proton scattering by measuring cross sections, beam-target asymmetry, or recoil polarization. Separate determinations of G_E/G_M by cross sections and recoil polarization observables disagree for Q²>1 (GeV/c)². Measurement by a third technique might uncover an unknown systematic error in either of the previous measurements. The beam-target asymmetry has been measured for elastic electron-proton scattering at Q² = 1.51 (GeV/c)² for target spin orientation aligned perpendicular to the beam momentum direction. This is the largest Q² at which G_E/G_M has been determined by a beam-target asymmetry experiment. The result, μG_E/G_M=0.884±0.027±0.029, is compared to previous world data.

The recoil proton polarization was measured in the ²H(e→,e^′p→)n reaction in Hall A of the Thomas Jefferson National Accelerator Facility. The electron kinematics were centered on the quasielastic peak (x_Bj≈1) and included three values of the squared four-momentum transfer, Q²=0.43,1.00 and 1.61 (GeV/c)². For Q²=0.43 and 1.61 (GeV/c)², the missing momentum, p_m, was centered at zero, whereas for Q²=1.00  (GeV/c)² two values of p_m were chosen: 0 and 174  MeV/c. At low p_m, the Q² dependence of the longitudinal polarization, P_z^′, is not well described by a state-of-the-art calculation. Further, at higher p_m, a 3.5σ discrepancy was observed in the transverse polarization, P_x^′. Understanding the origin of these discrepancies is important to confidently extract the neutron electric form factor from the analogous ²H(e→,e^′n→)p experiment.

A new six-parametric, axisymmetric, and asymptotically flat exact solution of Einstein-Maxwell field equations having reflection symmetry is presented. It has arbitrary physical parameters of mass, angular momentum, mass-quadrupole moment, current octupole moment, electric charge, and magnetic dipole, so it can represent the exterior field of a rotating, deformed, magnetized, and charged object; some properties of the closed-form analytic solution such as its multipolar structure, electromagnetic fields, and singularities are also presented. In the vacuum case, this analytic solution is matched to some numerical interior solutions representing neutron stars, calculated by Berti and Stergioulas [E. Berti and N. Stergioulas, Mon. Not. R. Astron. Soc. 350, 1416 (2004)], imposing that the multipole moments be the same. As an independent test of accuracy of the solution to describe exterior fields of neutron stars, we present an extensive comparison of the radii of innermost stable circular orbits (ISCOs) obtained from the Berti and Stergioulas numerical solutions, the Kerr solution [R. P. Kerr, Phys. Rev. Lett. 11, 237 (1963)], the Hartle and Thorne solution [J. B. Hartle and K. S. Thorne, Astrophys. J. 153, 807 (1968)], an analytic series expansion derived by Shibata and Sasaki [M. Shibata and M. Sasaki, Phys. Rev. D 58, 104011 (1998)], and our exact solution. We found that radii of ISCOs from our solution fits better than others with realistic numerical interior solutions.

Is there any time threshold of stability in a time-dependent hierarchical load-transfer structure formed by N elements, in the limit N→∞? There is no rigorous proof, but the consensus to this question is yes. Here we extend our previous work on these systems up to a size N=10⁶, using a power law breakdown rule and a new more efficient Monte Carlo method. The new results confirm this positive consensus.

We show the propagation of entangled states of high-dimensional quantum systems. The qudits states were generated using the transverse correlation of the twin photons produced by spontaneous parametric down-conversion. Their free-space distribution was performed at the laboratory scale and the propagated states maintained a high fidelity with their original form. The use of entangled qudits allow an increase in the quantity of information that can be transmitted and may also guarantee more privacy for communicating parties. Therefore, studies about propagating entangled states of qudits are important for the effort of building quantum communication networks.

We report an immediate generalization of the argument by Sen(De) and Sen proving that quantum dynamics, if linear, must be unitary, under the hypothesis of no-faster-than-light signaling between noninteracting entangled parties. The assumption of linearity is unnecessary and the conclusion is even stronger: not only are supraquantum cloning or deleting machines forbidden by the no signaling condition, but also any nonuniversal trace-preserving evolution.

The existence of a formal analogy between quantum energy spectra and discrete time series has been recently pointed out. When the energy level fluctuations are described by means of the δ_n statistic, it is found that chaotic quantum systems are characterized by 1∕f noise, while regular systems are characterized by 1∕f². In order to investigate the correlation structure of the δ_n statistic, we study the qth-order height-height correlation function C_q(τ), which measures the momentum of order q, i.e., the average qth power of the signal change after a time delay τ. It is shown that this function has a logarithmic behavior for the spectra of chaotic quantum systems, modeled by means of random matrix theory. On the other hand, since the power spectrum of chaotic energy spectra considered as time series exhibit 1∕f noise, we investigate whether the qth-order height-height correlation function of other time series with 1∕f noise exhibits the same properties. A time series of this kind can be generated as a linear combination of cosine functions with arbitrary phases. We find that the logarithmic behavior arises with great accuracy for time series generated with random phases.

We use neutron diffraction to probe the magnetization components of a crystal of Mn₁₂ single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S=10 and strong anisotropy along the crystallographic c axis. The application of a magnetic field H_⊥ perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T≲0.9(1)  K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for μ₀H_⊥≳5.5  T, possibly suggesting the existence of a quantum critical point.

The formation of plasma in wire-array Z-pinch experiments was found to depend upon the polarity of the radial-electric field near the wires. Reversing the radial-electric field midway along the length of an array resulted in the ablation rate of one-half of the array being reduced by 50%, significantly delaying the start of its implosion and altering its acceleration towards the axis. The observed phenomena cannot be explained by the standard magnetohydrodynamic models of array behavior, suggesting that effects such as electron emission may be important, especially during wire initiation.

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

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 ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.