Electroexcitation of Nucleon Resonances from CLAS Data on Single Pion Electroproduction
By I. Aznauryan, V. Burkert, The Clas Collaboration. 2009
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We present results on the electroexcitation of the low mass resonances Δ(1232)P 33 , N(1440)P 11 , N(1520)D 13 , and N(1535)S 11 in a wide range of Q 2 . The results were obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for π electroproduction off the proton. The data were analyzed using two conceptually different appioaches-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 Δ(1232)P 33 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 Q 2 ≤ 6 GeV 2 . For the Roper resonance, N(1440)P 11 , the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S 11 allowed us to obtain the branching ratios of this state to the πN and ηN channels via comparison with the results extracted from η electroproduction. The extensive CLAS data also enabled the extraction of the γ*p → N(1520)D 13 and N(1535)S 11 longitudinal helicity amplitudes with good precision. For the N(1535)S 11 , these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D 1 3 clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q 2 > 1 GeV 2 , confirming a long-standing prediction of the constituent quark model.
Subfields: Nuclear and High Energy Physics
View Paper: https://www.semanticscholar.org/paper/cce1fc599a04d240168b34e80be5661a35e952f8
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In Nuclear and High Energy Physics
Citations in the corpus
- Scattering processes and resonances from lattice QCD (2017, background): “…electromagnetic transition form factors (Segovia et al., 2015; Wilson et al., 2012), which can also be measured (Aznauryan and Burkert, 2012; Aznauryan et al., 2008, 2009; Dugger et al., 2009), could help to distinguish between different proposed scenarios, such as a quark orbital…”
- STUDIES OF NUCLEON RESONANCE STRUCTURE IN EXCLUSIVE MESON ELECTROPRODUCTION (2012, methodology): “(Left) TheA1/2 electrocoupling of theP11(1440) excited state from the analyses of the Nπ electroproduction data [17] (circles),ππp electroproduction data [13] (triangles), and preliminary results from theππp electroproduction data at Q(2) from 0.”
- Baryons as relativistic three-quark bound states (2016, methodology): “The study of hyperon structure is also one of the main goals of the CLAS collaboration.”