Meson Photo- and Electroproduction

Abstract

Photo- and electroproduction of pions and eta-mesons is studied on the nucleon. The formalism of such reactions is presented including polarization degrees of freedom and theoretical approaches are discussed. In particular, the threshold and $\Delta$-resonance region of pion photoproduction is considered and recent developments on low energy theorems (LET) and the E2/C2 quadrupole excitation of the $\Delta$ are presented. For eta photoproduction we show our calculations for the threshold region of the new Mainz experiment and demonstrate the sensitivity of this reaction to the elementary $\eta NN$ coupling and the resonance parameters of the $S_{11}(1535)$. In nuclear applications we point out the importance and advantage of light nuclei as $d$, $^3H$, $^3He$ and $^4He$ for the isospin structure of the reactions as well as for medium effects of the nucleon resonances.