The $\pi^-p \rightarrow e^+e^- n$ and $\pi^+n \rightarrow e^+e^- p$
reaction cross sections are calculated below and in the vicinity
of the vector meson ($\rho^0$, $\omega$) production threshold.
These processes are largely responsible for the emission of
$e^+e^-$ pairs in pion-nucleus reactions and contribute to the
dilepton spectra observed in relativistic heavy ion collisions.
They are dominated by the decay of low-lying baryon resonances
into vector meson-nucleon channels. The vector mesons materialize subsequently
into $e^+e^-$ pairs. Using $\pi N\rightarrow \rho^0 N$ and
$\pi N \rightarrow \omega N$ amplitudes calculated in the
center of mass energy interval $1.4<\sqrt s <1.8$ GeV, we compute
the $\pi^-p \rightarrow e^+e^- n$ and $\pi^+n \rightarrow e^+e^- p$
reaction cross sections in these kinematics. Below the vector meson production threshold, the
$\rho^0 - \omega$ interference in the $e^+e^-$ channel
appears largely destructive for
the $\pi^-p \rightarrow e^+e^- n$ cross section and
constructive for the $\pi^+n \rightarrow e^+e^- p$ cross section.
The pion beam and the HADES detector
at GSI offer a unique possibility to measure these effects. Such data would provide
strong constraints on the coupling of vector meson-nucleon channels to low-lying baryon
resonances. |
