e+e- pair production from nucleon targets in the resonance region

Abstract

We discuss two processes, the $\pi N \to e^+e^- N$ and the $\gamma N \to e^+e^- N$ reactions, below and close to the vector meson production threshold ($1.40<\sqrt s <1.75$ GeV). The aim is to gain understanding of the coupling of vector fields (associated with $\rho^0$- and $\omega$-mesons) to low-lying baryon resonances. These couplings are not well-known and important for baryon structure studies and for dynamical descriptions of vector meson propagation in the nuclear medium. The $e^+e^-$ pair production amplitudes are determined by the $\pi N \to \rho^0 N$, $\pi N \to \omega N$, $\gamma N\to \rho^0 N$ and $\gamma N \to \omega N$ amplitudes supplemented by the Vector Meson Dominance assumption. The vector meson production amplitudes are calculated consistently using a relativistic and unitary coupled-channel approach to meson-nucleon scattering. We display results showing the importance of the quantum interference between $\rho^0$- and $\omega$-mesons in the $e^+e^-$ channel to unravel the strength of the coupling of $\rho^0$- and $\omega$-mesons to specific baryon excitations. The $\pi N \to e^+e^- N$ and $\gamma N \to e^+e^- N$ reactions underlie the more complex $\pi A \to e^+e^- X$ and $\gamma A \to e^+e^- X$ nuclear processes whose measurement is planned at GSI (with the HADES detector) and under analysis at JLab (with the CLAS detector).