The π N ? e + e − N Reaction Close to the Vector-Meson Production Threshold

Abstract

The π− p→e + e − n and π+ n→e + e − p reaction cross sections are calculated below and in the vicinity of the vector-meson (ϱ0,ω) 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 πN→ϱ0 N and πN→ωN, amplitudes calculated in the center of mass energy interval 1.4 < √s<1.8 GeV, we compute the π− p→e + e − n and π+ n→e + e − p reaction cross sections in these kinematics. Below the vector-meson production threshold, the π0−ω interference in the e + e− channel appears largely destructive for the π− p→e + e − n cross section and constructive for the π+ n→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.