We discuss two processes, the $\\pi\\, N \\rightarrow e^+e^- \\,N$ and
the $\\gamma \\, N \\rightarrow e^+e^- \\,N$ 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 re\\-sonances. 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 $\\pi\\, N \\rightarrow \\rho^0 \\,N$, $\\pi\\, N \\rightarrow \\omega \\,N$,
$\\gamma \\,N\\rightarrow \\rho^0 N$ and
$\\gamma\\, N \\rightarrow \\omega N$ 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 $\\pi\\, N \\rightarrow e^+e^- \\,N$ and
$\\gamma \\, N \\rightarrow e^+e^- \\,N$ reactions underlie the more complex
$\\pi\\, A \\rightarrow e^+e^- \\, X$ and
$\\gamma \\, A \\rightarrow e^+e^- \\,X$ nuclear processes whose measurement is planned
at GSI (with the HADES detector) and under analysis at JLab (with the CLAS detector). |
