The Structure of the Nucleon: Elastic Electromagnetic Form Factors

Precise proton and neutron form factor measurements at Jefferson Lab, using spin observables, have recently made a significant contribution to the unraveling of the internal structure of the nucleon. Accurate experimental measurements of the nucleon form factors are a test-bed for understanding how the nucleon's static properties and dynamical behavior emerge from QCD, the theory of the strong interactions between quarks. There has been enormous theoretical progress, since the publication of the Jefferson Lab proton form factor ratio data, aiming at reevaluating the picture of the nucleon. We will review the experimental and theoretical developments in this field and discuss the outlook for the future.Comment: arXiv admin note: text overlap with arXiv:1301.0905, arXiv:hep-ph/0609004, arXiv:1411.6908 by other author

Separated Response Function Ratios in Exclusive, Forward pi(+/-) Electroproduction

The study of exclusive pi(+/-) electroproduction on the nucleon, including separation of the various structure functions, is of interest for a number of reasons. The ratio R-L=sigma(pi-)(L) / sigma(pi+)(L) is sensitive to isoscalar contamination to the dominant isovector pion exchange amplitude, which is the basis for the determination of the charged pion form factor from electroproduction data. A change in the value of R-T=sigma(pi-)(L) / sigma(pi+)(L) from unity at small -t, to 1/4 at large -t, would suggest a transition from coupling to a (virtual) pion to coupling to individual quarks. Furthermore, the mentioned ratios may show an earlier approach to perturbative QCD than the individual cross sections. We have performed the first complete separation of the four unpolarized electromagnetic structure functions above the dominant resonances in forward, exclusive p pi(+/-) electroproduction on the deuteron at central Q(2) values of 0.6, 1.0, 1.6 GeV2 at W=1.95 GeV, and Q(2)=2.45 GeV2 at W=2.22 GeV. Here, we present the L and T cross sections, with emphasis on R-L and R-T, and compare them with theoretical calculations. Results for the separated ratio R-L indicate dominance of the pion-pole diagram at low -t, while results for R-T are consistent with a transition between pion knockout and quark knockout mechanisms

Electroexcitation of the P33(1232), P11(1440), D13(1520), S11(1535) at Q^2=0.4 and 0.65(GeV/c)^2

Using two approaches: dispersion relations and isobar model, we have analyzed recent high precision CLAS data on cross sections of \pi^0, \pi^+, and \eta electroproduction on protons, and the longitudinally polarized electron beam asymmetry for p(\vec{e},e'p)\pi^0 and p(\vec{e},e'n)\pi^+. The contributions of the resonances P33(1232), P11(1440), D13(1520), S11(1535) to \pi electroproduction and S11(1535) to \eta electroproduction are found. The results obtained in the two approaches are in good agreement with each other. There is also good agreement between amplitudes of the \gamma^* N \to S11(1535) transition found in \pi and \eta electroproduction. For the first time accurate results are obtained for the longitudinal amplitudes of the P11(1440), D13(1520) and S11(1535) electroexcitation on protons.Comment: 9 pages, 9 figure

Electromagnetic proton form factors in large NcN_{c} QCD

The electromagnetic form factors of the proton are obtained using a particular realization of QCD in the large $N_c$ limit (${QCD}_{\infty}$), which sums up the infinite number of zero-width resonances to yield an Euler's Beta function (Dual-${QCD}_{\infty}$). The form factors $F_1(q^2)$ and $F_2(q^2)$, as well as $G_M(q^2)$ agree very well with reanalyzed space-like data in the whole range of momentum transfer. In addition, the predicted ratio $\mu_p G_E/G_M$ is in good agreement with recent polarization transfer measurements at Jefferson Lab.Comment: 10 page

Moderate acute alcohol intoxication has minimal effect on surround suppression measured with a motion direction discrimination task.

A well-studied paradox of motion perception is that, in order to correctly judge direction in high-contrast stimuli, subjects need to observe motion for longer in large stimuli than in small stimuli. This effect is one of several perceptual effects known generally as "surround suppression." It is usually attributed to center-surround antagonism between neurons in visual cortex, believed to be mediated by GABA-ergic inhibition. Accordingly, several studies have reported that this index of surround suppression is reduced in groups known to have reduced GABA-ergic inhibition, including older people and people with schizophrenia and major depressive disorder. In this study, we examined the effect on this index of moderate amounts of ethanol alcohol. Among its many effects on the nervous system, alcohol potentiates GABA-ergic transmission. We therefore hypothesized that it should further impair the perception of motion in large stimuli, resulting in a stronger surround-suppression index. This prediction was not borne out. Alcohol consumption slightly worsened duration thresholds for both large and small stimuli, but their ratio did not change significantly

Final analysis of proton form factor ratio data at Q(2)=4.0, 4.8, and 5.6 GeV2

Precise measurements of the proton electromagnetic form factor ratio R = mu(p)G(E)(p)/G(M)(p) using the polarization transfer method at Jefferson Lab have revolutionized the understanding of nucleon structure by revealing the strong decrease of R with momentum transfer Q(2) for Q(2) greater than or similar to 1 GeV2, in strong disagreement with previous extractions of R from cross-section measurements. In particular, the polarization transfer results have exposed the limits of applicability of the one-photon-exchange approximation and highlighted the role of quark orbital angular momentum in the nucleon structure. The GEp-II experiment in Jefferson Lab\u27s Hall A measured R at four Q(2) values in the range 3.5 GeV2 \u3c = Q(2) \u3c = 5.6 GeV2. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher Q(2) motivated a new analysis of the GEp-II data. This article presents the final results of the GEp-II experiment, including details of the new analysis, an expanded description of the apparatus, and an overview of theoretical progress since the original publication. The key result of the final analysis is a systematic increase in the results for R, improving the consistency of the polarization transfer data in the high-Q(2) region. This increase is the result of an improved selection of elastic events which largely removes the systematic effect of the inelastic contamination, underestimated by the original analysis

Bulk Mediated Surface Diffusion: Non Markovian Desorption with Finite First Moment

Here we address a fundamental issue in surface physics: the dynamics of adsorbed molecules. We study this problem when the particle's desorption is characterized by a non Markovian process, while the particle's adsorption and its motion in the bulk are governed by a Markovian dynamics. We study the diffusion of particles in a semi-infinite cubic lattice, and focus on the effective diffusion process at the interface $z = 1$. We calculate analytically the conditional probability to find the particle on the $z=1$ plane as well as the surface dispersion as functions of time. The comparison of these results with Monte Carlo simulations show an excellent agreement.Comment: 16 pages, 7 figs. European Physical Journal B (in press