Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

Journal Article · Sun Nov 15 00:00:00 EST 2009 · Physical Review. C, Nuclear Physics

DOI:https://doi.org/10.1103/PHYSREVC.80.055203· OSTI ID:21296530

Aznauryan, I G ^[1]; Burkert, V D; Avakian, H; Batourine, V; Carman, D S; Deur, A; Elouadrhiri, L; Guo, L; Kubarovsky, V; Weygand, D P ^[1]; Biselli, A S ^[2]; Egiyan, H ^[1]; Joo, K ^[3]; Kim, W; Kuznetsov, V; Stepanyan, S S ^[4]; Park, K ^[1]; Smith, L C; Prok, Y ^[5]; Ungaro, M ^[1]

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1. Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606 (United States)
2. Fairfield University, Fairfield, Connecticut 06824 (United States)
3. University of Connecticut, Storrs, Connecticut 06269 (United States)
4. Kyungpook National University, Daegu 702-701 (Korea, Republic of)
5. University of Virginia, Charlottesville, Virginia 22901 (United States)

We present results on the electroexcitation of the low mass resonances {delta}(1232)P{sub 33}, N(1440)P{sub 11}, N(1520)D{sub 13}, and N(1535)S{sub 11} in a wide range of Q{sup 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 {pi} electroproduction off the proton. The data were analyzed 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)P{sub 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{sup 2}{<=}6 GeV{sup 2}. For the Roper resonance, N(1440)P{sub 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{sub 11} allowed us to obtain the branching ratios of this state to the {pi}N and {eta}N channels via comparison with the results extracted from {eta} electroproduction. The extensive CLAS data also enabled the extraction of the {gamma}*p{yields}N(1520)D{sub 13} and N(1535)S{sub 11} longitudinal helicity amplitudes with good precision. For the N(1535)S{sub 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{sub 13} clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q{sup 2}>1 GeV{sup 2}, confirming a long-standing prediction of the constituent quark model.

• OSTI ID: 21296530
• Journal Information: Physical Review. C, Nuclear Physics, Vol. 80, Issue 5; Other Information: DOI: 10.1103/PhysRevC.80.055203; (c) 2009 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 0556-2813
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
AMPLITUDES
ASYMMETRY
BRANCHING RATIO
CEBAF ACCELERATOR
DELTA-1232 BARYONS
DIFFERENTIAL CROSS SECTIONS
ELECTRON BEAMS
ELECTROPRODUCTION
EXCITATION
GROUND STATES
MAGNETIC DIPOLES
N-1440 BARYONS
N-1520 BARYONS
N-1535 BARYONS
PIONS
POLARIZED BEAMS
PROTONS
QUARK MODEL
QUARKS
RESONANCE