Recoil Polarization Measurements for Neutral Pion Electroproduction at Q^2=1 (GeV/c)^2 Near the Delta Resonance

Abstract

We measured angular distributions of differential cross section, beam analyzing power, and recoil polarization for neutral pion electroproduction at Q^2 = 1.0 (GeV/c)^2 in 10 bins of W across the Delta resonance. A total of 16 independent response functions were extracted, of which 12 were observed for the first time. Comparisons with recent model calculations show that response functions governed by real parts of interference products are determined relatively well near 1.232 GeV, but variations among models is large for response functions governed by imaginary parts and for both increases rapidly with W. We performed a nearly model-independent multipole analysis that adjusts complex multipoles with high partial waves constrained by baseline models. Parabolic fits to the W dependence of the multipole analysis around the Delta mass gives values for SMR = (-6.61 +/- 0.18)% and EMR = (-2.87 +/- 0.19)% that are distinctly larger than those from Legendre analysis of the same data. Similarly, the multipole analysis gives Re(S0+/M1+) = (+7.1 +/- 0.8)% at W=1.232 GeV, consistent with recent models, while the traditional Legendre analysis gives the opposite sign because its truncation errors are quite severe. Finally, using a unitary isobar model (UIM), we find that excitation of the Roper resonance is dominantly longitudinal with S1/2 = (0.05 +/- 0.01) GeV^(-1/2) at Q^2=1. The ReS0+ and ReE0+ multipoles favor pseudovector coupling over pseudoscalar coupling or a recently proposed mixed-coupling scheme, but the UIM does not reproduce the imaginary parts of 0+ multipoles well.

60 pages, 54 figures

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• Investigation of the conjectured nucleon deformation at low momentum transfer
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• Electroproduction of nucleon resonances
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• Intrinsic quadrupole moment of the nucleon
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• A polarization relation and the measurement of the longitudinal response in pseudoscalar meson electroproduction off the nucleon
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Cited by in corpus (27)

• Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction
• Studies of Nucleon Resonance Structure in Exclusive Meson Electroproduction
• Electroexcitation of nucleon resonances
• 50 Years of Quantum Chromodynamics
• Electromagnetic Excitation of Nucleon Resonances
• Nucleon electromagnetic form factors and electroexcitation of low lying nucleon resonances in a light-front relativistic quark model
• Hadron structure at low Q^2
• Jülich-Bonn-Washington Model for Pion Electroproduction Multipoles
• Large-$N_c$ relations for the electromagnetic N to Delta(1232) transition
• Electromagnetic Transition Form Factors of Baryon Resonances
• Nucleon resonance contributions to unpolarised inclusive electron scattering
• Results from the N* program at JLab
• Electroexcitation of nucleon resonances of the [70,1-] multiplet in a light-front relativistic quark model
• Amplitude reconstruction from complete electroproduction experiments and truncated partial-wave expansions
• Improved empirical parametrizations of the ${\gamma }^{\ast }N\to \Delta (1232)$ and ${\gamma }^{\ast }N\to N(1520)$ helicity amplitudes and the Siegert's theorem
• Parametrizations of the ${\gamma }^{\ast }N\to \Delta (1232)$ quadrupole form factors and Siegert's theorem
• Electroexcitation of the Delta(1232)3/2+ and Delta(1600)3/2+ in a light-front relativistic quark model
• Exclusive ${\mathbf{\pi }}^0\mathbf{p}$ electroproduction off protons in the resonance region at photon virtualities 0.4~GeV${}^2$ $\le {\mathbf{Q}}^2\le 1$~GeV${}^2$
• Virtual Compton Scattering measurements in the ${\gamma }^{\ast }N\to \Delta$ transition
• Baryon transition form factors at the pole
• New low-$Q^2$ measurements of the ${\gamma }^{\ast }N\to \Delta (1232)$ Coulomb quadrupole form factor, pion cloud parametrizations and Siegert's theorem
• Ejectile Polarization for ${}^{2}H(e,e^{\prime }\vec{p})n$ at GeV energies
• Nucleon-to-Delta transition form factors in chiral effective field theory using the complex-mass scheme
• Electromagnetic transition form factors, $E2/M1$ and $C2/M1$ ratios of the baryon decuplet
• Combined parametrization of the neutron electric form factor and the ${\gamma }^{\ast }N\to \Delta (1232)$ quadrupole form factors
• Pion electroproduction measurements in the nucleon resonance region
• Axial-vector nucleon-to-delta transition form factors using the complex-mass renormalization scheme