Electromagnetic form factors of the transition from the Delta to the nucleon

Abstract

The low-energy electromagnetic form factors of the $\Delta$(1232)-to-nucleon transition are derived combining dispersion theory techniques and chiral perturbation theory. The form factors are expressed in terms of the well-understood pion vector form factor and pion-baryon scattering amplitudes. Nucleon and Delta exchange terms and contact terms constitute the input for these pion-baryon amplitudes. The framework is formulated for all form factors. When comparing to experimental data in the spacelike region of $e^- N \to e^- \Delta$ scattering, the focus lies on the numerically dominant magnetic dipole transition form factor. Fitting two subtraction constants (one for the scattering amplitude, one for the form factor) yields a very good description of this dominant form factor up to photon virtualities of about 0.6 GeV. After determining the subtraction constants in the spacelike region and at the photon point, respectively, predictions for the timelike region of Dalitz decays $\Delta \to N \, e^+ e^-$ are presented.