The π N → e +e−N reaction close to the vector-meson production threshold |
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Authors: | Madeleine Soyeur Matthias F. M. Lutz Bengt Friman |
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Affiliation: | 1. DAPNIA/SPhN, CEA/Saclay, F-91191, Gif-sur-Yvette Cedex, France 2. GSI, Planckstrasse 1, D-64291, Darmstadt, Germany 3. Institut für Kernphysik, TU Darmstadt, D-64289, Darmstadt, Germany
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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 (?0,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→?0 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. |
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