Spin effects and two-photon exchange in low-energy electron-nucleon scattering: New theoretical developments

The transverse single-spin asymmetry in inclusive electron-nucleon scattering, e + N(S_T)→ e’ + X, represents a pure two-photon exchange observable and is of fundamental interest for exploring higher-order QED effects in electron scattering. We compute this observable in the resonance region, where excitation of Δ isobars occurs in both intermediate and final states. We employ a novel theoretical method based on the large-N_c limit of QCD, which allows us to consistently combine N and Δ states and predict the elastic, inelastic, and inclusive spin-dependent cross section. Our results explain the contributions of N and Δ states and the N_c-scaling of the transverse spin asymmetry. Our predictions could be tested in future measurements of electron-nucleon scattering with polarized targets in the few-GeV energy range. Such experiments would complement earlier measurements of the inclusive single-spin asymmetry in the DIS regime (JLab, HERMES) and allow one to study the unknown dependence of two-photon exchange dynamics on the energy/momentum of the probe.