We review recent progress in the investigation of the electroexcitation of nucleon resonances, both in experiment and in theory. The most accurate results have been obtained for the electroexcitation ...amplitudes of the four lowest excited states, which have been measured in a range of Q2 up to 8 and 4.5 GeV2 for the Delta(1232)P33, N(1535)S11 and N(1440)P11, N(1520)D13}, respectively. These results have been confronted with calculations based on lattice QCD, large-Nc relations, perturbative QCD (pQCD), and QCD-inspired models. The amplitudes for the Delta(1232) indicate large pion-cloud contributions at low Q2 and don't show any sign of approaching the pQCD regime for Q2<7 GeV2. Measured for the first time, the electroexcitation amplitudes of the Roper resonance, N(1440)P11, provide strong evidence for this state as a predominantly radial excitation of a three-quark (3q) ground state, with additional non-3-quark contributions needed to describe the low Q2 behavior of the amplitudes. The longitudinal transition amplitude for the N(1535)S11 was determined and has become a challenge for quark models. Explanations may require large meson-cloud contributions or alternative representations of this state. The N(1520)D13 clearly shows the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q2 > 0.5 GeV2, confirming a long-standing prediction of the constituent quark model. The interpretation of the moments of resonance transition form factors in terms of transition transverse charge distributions in infinite momentum frame is presented.
Purpose. A comparative study of vicryl sutures 6-0 and 7-0 durability.Material and methods. The study was conducted in 20 enucleated pig eyes, divided into 2 groups (10 eyes each). We used vicryl ...suture 7-0 in the main group and 6-0 for control eyes. After surgical recession of the medial rectus we used the INSTRON 3382 dual column floor model testing system to access extensibility, increasing suture strain. At the moment of the first suture rupture we measured strain and muscle elongation.Results. On average, vicryl suture 7-0 endured half the strain and muscle elongation before the rupture than vicryl 6-0. But this kind of stress reduction is not critical for the overall result, because to tear a muscle sutured with a fine thread, extraocular muscles would still need 4 times the force they naturally possess.Conclusion. Fine vicryl 7-0 thread possesses the necessary characteristics for oculomotor muscles surgery.
Transition helicity amplitudes \(\gamma_{v}NN^*\) (electrocouplings) were determined for prominent excited proton states with masses below 1.8 GeV in independent analyses of major meson ...electroproduction channels: \(\pi^{+}n\), \(\pi^0p\) and \(\pi^+\pi^-p\). Consistent results on resonance electrocouplings obtained from analyses of these exclusive reactions with different non-resonant contributions demonstrate reliable extraction of these fundamental quantities for states that have significant decays for either \(N\pi\) or \(N\pi\pi\) channels. Preliminary results on electrocouplings of \(N^*\) states with masses above 1.6 GeV have become available from the CLAS data on \(\pi^+\pi^-p\) electroproduction off protons for the first time. Comparison with quark models and coupled-channel approaches strongly suggest that \(N^*\) structure is determined by contributions from an internal core of three constituent quarks and an external meson-baryon cloud at the distances covered in these measurements with the CLAS detector.
We present results on the electroexcitation of the low mass resonances Delta(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11 in a wide range of Q2. The results were obtained in the comprehensive ...analysis of JLab-CLAS data on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for pion electroproduction off the proton. The data were analysed using two conceptually different approaches, fixed-t dispersion relations and a unitary isobar model, allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Delta(1232)P33} show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q2<6 GeV2. For the Roper resonance, N(1440)P11, the data provide strong evidence for this state as a predominantly radial excitation of a 3-quark ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S11 allowed us to obtain the branching ratios of this state to the piN and etaN channels via comparison to the results extracted from eta electroproduction. The extensive CLAS data also enabled the extraction of the gamma*p -> N(1520)D13 and N(1535)S11 longitudinal helicity amplitudes with good precision.
We present definitions and formulas that may be useful for a consistent computation of helicity amplitudes for the process gamma* N -> N* in theoretical approaches. Of particular importance is the ...correct determination of the common sign of the amplitudes and of the relative sign between the transverse (A12,A32) and longitudinal (S12) amplitudes. This clarification is necessary to clear up confusions present in theoretical works. Using the definitions presented in this paper will enable a direct comparison with amplitudes extracted from experimental data.
Studies of the structure of excited baryons are key to the N* program at Jefferson Lab. Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated ...experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever yet achieved, with a kinematic reach up to Q2 = 12 GeV2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with QCD-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the non-perturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.
The helicity amplitudes of the electroexcitation of the Roper resonance are extracted for 1.7 < Q2 < 4.5 GeV2 from recent high precision JLab-CLAS cross section and longitudinally polarized beam ...asymmetry data for pi+ electroproduction on protons at W=1.15-1.69 GeV. The analysis is made using two approaches, dispersion relations and a unitary isobar model, which give consistent results. It is found that the transverse helicity amplitude A_{1/2} for the gamma* p -> N(1440)P11 transition, which is large and negative at Q2=0, becomes large and positive at Q2 ~ 2 GeV2, and then drops slowly with Q2. The longitudinal helicity amplitude S_{1/2}, which was previously found from CLAS ep -> eppi0,enpi+ data to be large and positive at Q2=0.4,0.65 GeV2, drops with Q2. Available model predictions for gamma* p -> N(1440)P11 allow us to conclude that these results provide strong evidence in favor of N(1440)P11 as a first radial excitation of the 3q ground state. The results of the present paper also confirm the conclusion of our previous analysis for Q2 < 1 GeV2 that the presentation of N(1440)P11 as a 3qG hybrid state is ruled out.
We report a new measurement of the exclusive electroproduction reaction {gamma}*p{yields}{pi}{sup 0}p to explore the evolution from soft nonperturbative physics to hard processes via the Q{sup 2} ...dependence of the magnetic (M{sub 1+}), electric (E{sub 1+}), and scalar (S{sub 1+}) multipoles in the N{yields}{delta} transition. 9000 differential cross section data points cover W from threshold to 1.4 GeV/c{sup 2}, 4{pi} center-of-mass solid angle, and Q{sup 2} from 3 to 6 GeV{sup 2}/c{sup 2}, the highest yet achieved. It is found that the magnetic form factor G{sub M}* decreases with Q{sup 2} more steeply than the proton magnetic form factor, the ratio E{sub 1+}/M{sub 1+} is small and negative, indicating strong helicity nonconservation, and the ratio S{sub 1+}/M{sub 1+} is negative, while its magnitude increases with Q{sup 2}.
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Phys. Rev. C 92, 015203 (2015) We report on the determination of the electrocouplings for the transition
from the proton to the $N(1675){\frac{5}{2}}^-$ resonance state using recent
differential ...cross section data on $e p \rightarrow e\pi^+ n$ by the CLAS
collaboration at $1.8 \le Q^2 < 4.5$GeV$^2$. The data have been analyzed using
two different approaches, the unitary isobar model and fixed-t dispersion
relations. The extracted $\gamma^* p\rightarrow N(1675){\frac{5}{2}}^-$
helicity amplitudes show considerable coupling through the $A^p_{1/2}$
amplitude, that is significantly larger than predicted three-quark contribution
to this amplitude. The amplitude $A^p_{3/2}$ is much smaller. Both results are
consistent with the predicted sizes of the meson-baryon contributions at $Q^2
\geq 1.8 $GeV$^2$ from the dynamical coupled-channel model.
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