Threshold Pion Electroproduction in Chiral Perturbation Theory

Electroproduction of pions on the nucleon near the threshold is analyzed within the framework of baryon chiral perturbation theory. We give a thorough discussion of the low–energy theorems related to charged and neutral electropionproduction. It is shown how the axial radius of the nucleon can be related to the S–wave multipoles $E_{0+}^{(-)}$ and $L_{0+}^{(-)}$. The chiral perturbation theory calculations of the ${\gamma }^{\star }p\to {\pi }^{0}p$ reaction are found to be in good agreement with the recent near threshold data. We also discuss the influence of some isospin–breaking effects in this channel. For future experimental tests of the underlying chiral dynamics, extensive predictions of differential cross sections and multipole amplitudes are presented.