Exposing Nucleon Resonance Structure using Electron and Pion Beams*

Contributing to the effort of exploring the connection of the physics associated with electron beams and pion beams, we study the electromagnetic transitions between the nucleon and the nucleon resonances N* in terms of the invariant square momentum transfer q2. The analysis is divided formally into the spacelike region (probed by electron beams) with the determination of the electromagnetic transition form factors, and the timelike region (probed by pion beams) with the calculation of the nucleon resonance Dalitz decay (N* -> e+e- N) rates.

Experiments with pion probes at HADES (GSI) cover at the moment the range associated with the Delta(1232) and the second resonance region. We present our estimates for the Delta(1232), N(1520) and N(1535) Dalitz decay rates for proton or neutron targets. Our results are compared with the available data from HADES for those nucleon resonances.

The calculations are performed in a covariant model based on the valence quark degrees of freedom, originally developed for the spacelike region. The model combines the effects associated to the quarks (bare contribution), with effective parametrizations of the meson cloud effects according to theoretical and empirical constraints. The formalism is extended to the timelike region considering the natural kinematic extrapolation of the bare contributions, and analytic continuations of the meson cloud contributions compatible with the timelike regime.

Our estimates of the electromagnetic structure in the timelike region can be used in the analysis of future experiments. We discuss also the possibility of extending formalism for other nucleon resonances.