Search Results (20 total)

Suprathreshold stochastic resonance (SSR) is a form of noise-enhanced signal transmission that occurs in a parallel array of independently noisy identical threshold nonlinearities, including model neurons. Unlike most forms of stochastic resonance, the output response to suprathreshold random input signals of arbitrary magnitude is improved by the presence of even small amounts of noise. In this paper, the information transmission performance of SSR in the limit of a large array size is considered. Using a relationship between Shannon’s mutual information and Fisher information, a sufficient condition for optimality, i.e., channel capacity, is derived. It is shown that capacity is achieved when the signal distribution is Jeffrey’s prior, as formed from the noise distribution, or when the noise distribution depends on the signal distribution via a cosine relationship. These results provide theoretical verification and justification for previous work in both computational neuroscience and electronics.

The data analysis for the reaction ¹H(e,e^'π⁺)n, which was used to determine values for the charged pion form factor F_π for values of Q²= 0.6–1.6 GeV², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract F_π from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of F_π are presented.

Electroproduction of the ω meson was investigated in the ¹H(e,e^′p)ω reaction. The measurement was performed at a four-momentum transfer Q²≈0.5  GeV². Angular distributions of the virtual photon-proton center-of-momentum cross sections have been extracted over the full angular range. These distributions exhibit a strong enhancement over t-channel parity exchange processes in the backward direction. According to a newly developed electroproduction model, this enhancement provides significant evidence of resonance formation in the γ*p→ωp reaction channel.

We report the results from a systematic study of the quasielastic (e,e^′p) reaction on ¹²C,  ⁵⁶Fe, and ¹⁹⁷Au performed at Jefferson Lab. We have measured nuclear transparency and extracted spectral functions (corrected for radiation) over a Q² range of 0.64–3.25  (GeV∕c)² for all three nuclei. In addition, we have extracted separated longitudinal and transverse spectral functions at Q² of 0.64 and 1.8  (GeV∕c)² for these three nuclei (except for ¹⁹⁷Au at the higher Q²). The spectral functions are compared to a number of theoretical calculations. The measured spectral functions differ in detail but not in overall shape from most of the theoretical models. In all three targets the measured spectral functions show considerable excess transverse strength at Q²=0.64  (GeV∕c)², which is much reduced at 1.8  (GeV∕c)².

We develop an analytically solvable three-state discrete-time minimal Brownian ratchet (MBR), where the transition probabilities between states are asymmetric. By solving the master equations, we obtain the steady-state probabilities. Generally, the steady-state solution does not display detailed balance, giving rise to an induced directional motion in the MBR. For a reduced two-dimensional parameter space, we find the null curve on which the net current vanishes and detailed balance holds. A system on this curve is said to be balanced. On the null curve, an additional source of external random noise is introduced to show that a directional motion can be induced under the zero overall driving force.

We report measurements of cross sections for the reaction ¹H(e,e^′K⁺)Y, for both the Λ and Σ⁰ hyperon states, at an invariant mass of W=1.84 GeV and four-momentum transfers 0.5<Q²<2 (GeV/c)². Data were taken for three values of virtual photon polarization ε, allowing the decomposition of the cross sections into longitudinal and transverse components. The Λ data are a revised analysis of prior work, whereas the Σ⁰ results have not been previously reported.

A version of the Monty Hall problem is presented where the players are permitted to select quantum strategies. If the initial state involves no entanglement the Nash equilibrium in the quantum game offers the players nothing more than that obtained with a classical mixed strategy. However, if the initial state involves entanglement of the qutrits of the two players, it is advantageous for one player to have access to a quantum strategy while the other does not. Where both players have access to quantum strategies there is no Nash equilibrium in pure strategies, however, there is a Nash equilibrium in quantum mixed strategies that gives the same average payoff as the classical game.

Separated longitudinal and transverse structure functions for the reaction ¹H(e,e^′π⁺)n were measured in the momentum transfer region Q²  =  0.6–1.6 (GeV/c)² at a value of the invariant mass W  =  1.95 GeV. New values for the pion charge form factor were extracted from the longitudinal cross section by using a recently developed Regge model. The results indicate that the pion form factor in this region is larger than previously assumed and is consistent with a monopole parametrization fitted to very low Q² elastic data.

Based on Brownian ratchets, a counterintuitive phenomenon has recently emerged—namely, that two losing games can yield, when combined, a paradoxical tendency to win. A restriction of this phenomenon is that the rules depend on the current capital of the player. Here we present new games where all the rules depend only on the history of the game and not on the capital. This new history-dependent structure significantly increases the parameter space for which the effect operates.

Tensor polarization observables ( t₂₀, t₂₁, and t₂₂) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)². The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q² the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.

A separation of the longitudinal and transverse ¹²C(e,e^′p) cross sections in the quasifree region has been performed in parallel kinematics at Q² of 0.64 and 1.8 GeV² for initial proton momentum <80 MeV. The separated transverse and longitudinal spectral functions at Q²=0.64 GeV² show significant differences for missing energy between 25 and 60 MeV indicating a breakdown in the single nucleon knockout picture. The transverse spectral functions exhibit definite momentum transfer dependence.

The differential cross section for ²H(γ,d)π⁰ has been measured at deuteron center-of-mass angles of 90° and 136°. This work reports the first data for this reaction above a photon energy of 1 GeV, and permits a test of the apparent constituent counting rule and reduced nuclear amplitude behavior as observed in elastic ed scattering. Measurements were performed up to a photon energy of 4.0 GeV, and are in good agreement with previous lower energy measurements. Overall, the data are inconsistent with both constituent-counting rule and reduced nuclear amplitude predictions.

Muonic x-ray cascades in B, C, N, O, and Ne following muonic atom formation in B₂H₆, CH₄, C₂H₆, C₄H₁₀, N₂, O₂, and Ne were investigated. The densities of the different target gases were low enough to prevent any contact of the atom or molecule on which the formation takes place with surrounding atoms or molecules during the cascade. Using semiconductor detectors, the yields of transitions in several series with energies between 1.5 keV and 300 keV were measured. Cascade calculations were performed in order to explain the observed transition yields. The cascades could be reproduced by variation of only two parameters: the number of electrons and the muon angular momentum distribution at the starting point of the calculation. The cascade of μNe formed in monoatomic neon was described best. Moreover, muonic carbon cascades were found to differ according to the sort of hydrocarbon, demonstrating the influence of the capturing molecule’s structure on the muonic cascade. By varying the number of initially available electrons, the molecular effects could be described with the cascade program.

The A(Q²) structure function in elastic electron-deuteron scattering was measured at six momentum transfers Q² between 0.66 and 1.80 (GeV/c)² in Hall C at Jefferson Laboratory. The scattered electrons and recoil deuterons were detected in coincidence, at a fixed deuteron angle of 60.5°. These new precise measurements resolve discrepancies between older sets of data. They put significant constraints on existing models of the deuteron electromagnetic structure, and on the strength of isoscalar meson exchange currents.

The first measurements of the differential cross section for the d(γ,p)n reaction up to 4.0 GeV were performed at the Continuous Electron Beam Accelerator Facility (CEBAF) at Thomas Jefferson Laboratory. We report the cross sections at the proton center-of-mass angles of 36°, 52°, 69°, and 89°. These results are in reasonable agreement with previous measurements at lower energy. The 89° and 69° data show constituent-counting-rule behavior up to 4.0 GeV photon energy. The 52° and 36° data disagree with the counting-rule behavior. The quantum chromodynamics (QCD) model of nuclear reactions involving reduced amplitudes disagrees with the present data.

The ¹H(e,e^′K⁺)Λ reaction was studied as a function of the squared four-momentum transfer, Q², and the virtual photon polarization, ɛ. For each of four Q² settings, 0.52, 0.75, 1.00, and 2.00 (GeV/c)², the longitudinal and transverse virtual photon cross sections were extracted in measurements at three virtual photon polarizations. The Q² dependence of the σ_L/σ_T ratio differs significantly from current theoretical predictions. This, combined with the precision of the measurement, implies a need for revision of existing calculations.

The ( e,e^′p) reaction was studied on targets of C, Fe, and Au at momentum transfers squared Q² of 0.6, 1.3, 1.8, and 3.3 GeV² in a region of kinematics dominated by quasifree electron-proton scattering. Missing energy and missing momentum distributions are reasonably well described by plane wave impulse approximation calculations with Q² and A dependent corrections that measure the attenuation of the final state protons.

The cascade of muonic boron and the two-photon decay 2S→1S have been investigated. The x-ray measurement with Ge and Si(Li) detectors was performed at low target gas pressures of pure B₂H₆ and admixtures with He. The 2S population of μB is found to be ε_2S  =  3.1(2)%. Cascade calculations indicate that more than half of the 2S states are populated with an empty electron shell. Two-photon decays have been observed in mixtures of B₂H₆ in He at partial diborane pressures of 67 Pa and below. The measured fraction of metastable 2S states per μB atom is 0.58(16)% in an admixture of 1% B₂H₆ in He at 6.7 kPa and is interpreted in terms of pressure dependent quenching.

Diffusion of muonic deuterium μd and muonic hydrogen μp atoms produced following the stopping of negative muons in D₂ or H₂ at 300 K was studied at pressures of 47–750 mbar (H₂) and 94–1520 mbar (D₂) in two distinct target geometries. Time intervals were recorded between entry of negative muons into the gas and arrival of each resulting μd or μp atom at one of 50 foils immersed in the gas, and spaced regularly along the muon beam axis. The results of such measurements were fitted to time distributions generated by Monte Carlo methods, using theoretical scattering predictions and empirically chosen forms for the initial energy distributions of the muonic atoms in the 1S state. Results indicate muonic atom energy distributions which (a) are different for μd and μp and (b) vary with pressure. The best-fit energy distributions have mean energies ranging from 1.5 eV for μd at 94 mbar to ≥9 eV for μp at 750 mbar. The data are also sensitive to scattering cross sections for μd and μp, and are consistent with current theoretical calculations for the μd+D₂ cross sections. In the case of μp+H₂ scattering, the experimental data suggest discrepancies with the theoretical predictions.

Elastic and inelastic scattering cross sections and analyzing powers have been determined for 16 MeV polarized protons incident on ¹¹⁶,120,124Sn. Data extracted for these isotopes include scattering from the lowest 2⁺ and 3^- collective states and two other less collective states (5^-,4⁺) below 3 MeV in excitation. Our analysis of the data uses a coupled-channel formalism based on the harmonic vibrational model with a (0⁺,2⁺,3^-,5^-4⁺) coupling scheme. Nuclear deformation parameters of the central and spin-orbit potentials for the excited states have been extracted and compared with similar quantities deduced from (n→,n) scattering from the same isotopes.