Electroexcitation of nucleon resonances at Q^2=0.65 GeV/c^2 from a combined analysis of single- and double-pion electroproduction data

Data on single- and double-charged pion electroproduction off protons are successfully described in the second and third nucleon resonance regions with common N* photocouplings. The analysis was carried out using separate isobar models for both reactions. From the combined analysis of two exclusive channels, the gamma* p --> N*+ helicity amplitudes are obtained for the resonances P11(1440), D13(1520), S31(1620), S11(1650), F15(1680), D33(1700), D13(1700), and P13(1720) at Q2=0.65 GeV/c^2.Comment: 12 pages, 12 figures (eps), Published in PHYSICAL REVIEW C 72, 045201 (2005

Model Analysis of the ep→ep′π+π−ep \to ep'\pi^+\pi^- Electroproduction Reaction on the Proton

Recent CLAS data on the $p\pi^+\pi^-$ electroproduction off protons at 1.3$<$W$<$1.57 GeV and 0.25$<$$Q^{2}$$<$0.6 GeV$^{2}$ have been analyzed using a meson-baryon phenomenological model. By fitting nine 1-fold differential cross section data for each $W$ and $Q^{2}$ bin, the charged double pion electroproduction mechanisms are identified from their manifestations in the observables. We have extracted the cross sections from amplitudes of each of the considered isobar channels as well as from their coherent sum. We also obtained non-resonant partial wave amplitudes of all contributing isobar channels which could be useful for advancing a complete coupled-channel analysis of all meson electroproduction data.Comment: Experiment Numbers: E93-006, E94-005 Group: Hall

Nucleon Resonance Electrocouplings from the CLAS Meson Electroproduction Data

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.Comment: 6 pages, 3 figure

Polarized proton structure in the resonance region

In view of the precise data available on inclusive polarized electron scattering off polarized proton targets in the nucleon resonance excitation region, we compare these results with the coherent sum of resonant contributions to the polarized structure function $g_1$ and virtual photon asymmetry $A_1$. To this goal, we employ the nucleon resonance electroexcitation amplitudes determined for photon virtualities $Q^2$ $<$ 5.0 GeV$^2$ from analyses of the CLAS data on exclusive electroproduction off protons in the resonance region. Most of the well established resonances of four star PDG status in the mass range up to 1.75~GeV are included. We find that the resonance-like structures observed in the inclusive $g_1$ data are related to the resonant contributions in the entire range of photon virtuality $Q^2$ where the data on $g_1$ are available. In the range of invariant mass of the final hadron system $W$ $>$ 1.5 GeV, the data on the asymmetry $A_1$ are well reproduced even when accounting for resonant contributions only, especially for the larger values of $Q^2$ and energies analysed. This observation offers an interesting hint to quark-hadron duality seen in polarized inclusive electron scattering observables.Comment: Contribution to proceedings: Emergence and Structure of Baryons - Selected Contributions from the International Conference Baryons 202

Experimental Study of the \u3cem\u3eP\u3c/em\u3e\u3csub\u3e11\u3c/sub\u3e(1440) and \u3cem\u3eD\u3c/em\u3e\u3csub\u3e13\u3c/sub\u3e(1520) Resonances from the CLAS Data on \u3cem\u3eep→e′π+π−p′\u3c/em\u3e

The transition helicity amplitudes from the proton ground state to the P11(1440) and D13(1520) excited states (vγpN* electrocouplings) were determined from the analysis of nine independent one-fold differential π+π−p electroproduction cross sections off a proton target, taken with CLAS at photon virtualities of 0.25 \u3c Q2 \u3c 0.60 GeV2. The phenomenological reaction model was employed for separation of the resonant and nonresonant contributions to the final state. The P11(1440) and D13(1520) electrocouplings were obtained from the resonant amplitudes parametrized within the framework of a unitarized Breit-Wigner ansatz. They are consistent with results obtained in the previous CLAS analyses of the π+n and π0p channels. The successful description of a large body of data in dominant meson-electroproduction channels off protons with the same γvpN* electrocouplings offers clear evidence for the reliable extraction of these fundamental quantities from meson-electroproduction data. This analysis also led to the determination of the long-awaited hadronic branching ratios for the D13(1520) decay into Δπ(24%–32%) and Nρ (8%–17%)