Nucleon Resonance Excitation with Virtual Photons

Abstract

The unitary isobar model MAID is used for a partial wave analysis of pion photoproduction and electroproduction data on the nucleon. In particular we have taken emphasis on the region of the Delta(1232) resonance and have separated the resonance and background amplitudes with the K-matrix approach. This leads to electromagnetic properties of the dressed Delta resonance, where all multipole amplitudes become purely imaginary and all form factors and helicity amplitudes become purely real at the K-matrix pole of W=M_Delta=1232 MeV. The REM=E2/M1 and RSM=C2/M1 ratios of the quadrupole excitation are compared to recent data analysis of different groups. The REM ratio of MAID2005 agrees very well with the data and has a linear behavior over the whole experimentally explored Q^2 region with a small positive slope that predicts a zero crossing around 3.5 GeV^2. The recent RSM data for Q^2 < 0.2 GeV^2 indicate a qualitative change in the shape of the ratio which can be explained by the impact of the Siegert theorem at pseudothreshold (Q^2=-0.086 GeV^2) in the unphysical region.

arXiv comment12 pages, 7 figures, Workshop Shape of Hadrons, Athens, Greece, 2006

Cited by in corpus (9)

• D-state effects in the electromagnetic N-Delta transition
• Transition form factors of the N*(1535) as a dynamically generated resonance
• Electromagnetic Transition Form Factors of Baryon Resonances
• 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
• $N^{\ast }$ Form Factors based on a Covariant Quark Model
• Improved empirical parametrizations of the ${\gamma }^{\ast }N\to N(1535)$ transition amplitudes and the Siegert's theorem
• New low-$Q^2$ measurements of the ${\gamma }^{\ast }N\to \Delta (1232)$ Coulomb quadrupole form factor, pion cloud parametrizations and Siegert's theorem
• Analytic parametrizations of the ${\gamma }^{\ast }N\to N(1440)$ form factors inspired by Light-Front holography
• Combined parametrization of the neutron electric form factor and the ${\gamma }^{\ast }N\to \Delta (1232)$ quadrupole form factors