Helicity Amplitudes and Sum Rules for Real and Virtual Photons

The spin structure of the nucleon in the resonance region is of particular interest to understand the rapid transition from resonance dominated coherent processes to incoherent processes of deep inelastic scattering (DIS) off the constituents. Here we present the results of the recently developed unitary isobar model (MAID) for the spin asymmetries, structure functions and relevant sum rules in the resonance region. This model describes the presently available data for single-pion photoand electroproduction up to a total cm energy Wmax = 2 GeV and for Q 2 ≤ 4 (GeV/c). It is based on effective Lagrangians for Born terms and vector meson exchange (background) and resonance contributions modeled by Breit-Wigner functions. All major resonances below W = 1700 MeV are included. The respective multipoles are constructed in a gauge-invariant and unitary way for each partial wave. The eta production is included in a similar way, while the contribution of more-pion and higher channels is modeled by comparison with the total cross sections and simple phenomenological assumptions.