Lowest Q^2 Measurement of the gamma*p -> Delta Reaction: Probing the Pionic Contribution

Abstract

To determine nonspherical angular momentum amplitudes in hadrons at long ranges (low Q^2), data were taken for the p(\vec{e},e'p)π^0 reaction in the Delta region at Q^2=0.060 (GeV/c)^2 utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W=1232 MeV are: M_{1+}^{3/2} = (40.33 +/- 0.63_{stat+syst} +/- 0.61_{model}) (10^{-3}/m_{π^+}),Re(E_{1+}^{3/2}/M_{1+}^{3/2}) = (-2.28 +/- 0.29_{stat+syst} +/- 0.20_{model})%, and Re(S_{1+}^{3/2}/M_{1+}^{3/2}) = (-4.81 +/- 0.27_{stat+syst} +/- 0.26_{model})%. These disagree with predictions of constituent quark models but are in reasonable agreement with lattice calculations with non-linear (chiral) pion mass extrapolations, with chiral effective field theory, and with dynamical models with pion cloud effects. These results confirm the dominance, and general Q^2 variation, of the pionic contribution at large distances.

arXiv comment6 pages, 3 figures, 1 table

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• Unitary Isobar Model - MAID2007
• Electromagnetic excitation of the Delta(1232)-resonance
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• Electroexcitation of nucleon resonances
• Electromagnetic Excitation of Nucleon Resonances
• Extraction and Interpretation of gamma N --> Delta Form Factors within a Dynamical Model
• The nucleon to Delta electromagnetic transition form factors in lattice QCD
• Nucleon electromagnetic form factors and electroexcitation of low lying nucleon resonances in a light-front relativistic quark model
• D-state effects in the electromagnetic N-Delta transition
• Cross sections and beam asymmetries for $\vev{e}p \to enπ^+$ in the nucleon resonance region for $1.7\le Q^2\le 4.5(GeV{)}^2$
• Nucleon to Delta transition form factors with $N_F=2+1$ domain wall fermions
• Valence quark contribution for the gamma N -> Delta quadrupole transition extracted from lattice QCD
• Delta and Omega electromagnetic form factors in a Dyson-Schwinger/Bethe-Salpeter approach
• Measurements of $ep\to e^{\prime }{\pi }^{+}n$ at W = 1.6 - 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS
• Extraction of Electromagnetic Transition Form Factors for Nucleon Resonances within a Dynamical Coupled-Channels Model
• Hadron structure at low Q^2
• Determination of quadrupole strengths in the gamma*p-->Delta(1232) transition at Q2= 0.20 (GeV/c)2
• Measurement of the neutron charge radius and the role of its constituents
• Measurements of the gamma*p->Delta Reaction At Low Q^2: Probing the Mesonic Contribution
• Delta-baryon mass in a covariant Faddeev approach
• 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
• Angular distributions in electroweak pion production off nucleons: odd parity hadron terms, strong relative phases and model dependence
• Dynamical Models of the Excitations of Nucleon Resonances
• Results from the N* program at JLab
• Neutral Pion Electroproduction in the Resonance Region at High $Q^2$
• Electromagnetic Nucleon-to-Delta Transition in Holographic QCD
• First measurement of target and double spin asymmetries for polarized e- polarized p --> e p pi0 in the nucleon resonance region above the Delta(1232)
• Electroexcitation of nucleon resonances of the [70,1-] multiplet in a light-front relativistic quark model
• Coupled-channel analysis of pion- and eta-electroproduction with the Jülich-Bonn-Washington model
• Study of nucleon resonances with electromagnetic interactions
• Electroexcitation of the ${\Delta }^{+}(1232)$ at low momentum transfer
• Measurements of the γ* p --> Δ(1232) reaction at low Q2
• Electroexcitation of the Delta(1232)3/2+ and Delta(1600)3/2+ in a light-front relativistic quark model
• Virtual Compton Scattering measurements in the ${\gamma }^{\ast }N\to \Delta$ transition
• Impact of JLab data on the determination of GPDs at zero skewness and new insights from transition form factors $N\to \Delta$
• Virtual Compton Scattering measurements in the nucleon resonance region
• Electroexcitation of nucleon resonances in a light-front relativistic quark model
• Electromagnetic transition form factors, $E2/M1$ and $C2/M1$ ratios of the baryon decuplet
• Q^2-evolution of nucleon-to-resonance transition form factors in a QCD-inspired vector-meson-dominance model
• Analysis of the quadrupole deformation of $\Delta$(1232) within an effective Lagrangian model for pion photoproduction from the nucleon
• Electromagnetic form factors of the Delta(1232) in Dual-Large N_c QCD
• Partial-Wave Analysis of Single-Pion Production Reactions
• Measurement of the Partial Cross Sections s(TT), s(LT) and [s(T)+epsilon*s(L)] of the p(e,e' pi+)n Reaction in the Delta(1232) Resonance
• Light-quark baryon spectroscopy from ANL-Osaka dynamical coupled-channels analysis
• Pion electroproduction measurements in the nucleon resonance region
• Electromagnetic properties of nucleons and hyperons in a Lorentz covariant quark model
• Multipole sensitivity to phase variation in pion photo-and electroproduction analyses