Nucleon structure from the quasithreshold inverse pion electroproduction

Abstract

The process $\pi N\to e^+e^- N$ (IPE), being a natural and unique laboratory for studying the hadron electromagnetic structure in the sub-$N\bar{N}$-threshold time-like region of the virtual-photon ``mass'' $\lambda^2$, turns out to be also very useful for investigating the nucleon weak structure. A theoretical basis of the methods for extracting practically model-independent values of the electromagnetic hadron form factors in the time-like region and for determining the weak structure of the nucleon in the space-like region from experimental data on IPE at low energies is outlined. The results of extracting, by those methods, the electromagnetic $F_1^v(\lambda^2)$ and pseudoscalar $G_P(t)$ form-factors of a nucleon are presented, where an indication of the existence of the state $\pi^\p$ in the range 500-800 MeV (possibly the first radial excitation of pion) is obtained, and the coupling constant of this new state with the nucleon is estimated.