e⁺e^? Pair Production from Nucleon Targets in the Resonance Region

摘要

We discuss two processes, the πN → e⁺e^?N and the γN → e⁺e^?N reactions, below and close to the vector meson production threshold (1.40 0- and ω-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 πN → ρ⁰N, πN → ωN, γN → ρ⁰N and γN → ωN 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 ρ⁰- and ω-mesons in the e⁺e^? channel to unravel the strength of the coupling of ρ⁰- and ω-mesons to specific baryon excitations. The πN → e⁺e^?N and γN → e⁺e^?N reactions underlie the more complex πA → e⁺e^?X and γA → e⁺e^?X nuclear processes whose measurement is planned at GSI (with the HADES detector) and under analysis at JLab (with the CLAS detector).