Meson-nucleon scattering and vector mesons in nuclear matter
The electromagnetic decay of the vector mesons into e+e− and μ+μ− pairs makes them particularly well suited for exploring the conditions in dense and hot matter in nuclear collisions. The lepton pairs provide virtually undistorted information on the mass distribution of the vector mesons in the medium. The lepton-pair spectrum in nucleus-nucleus collisions at SPS energies exhibits a low-mass enhancement compared to proton-proton and protonnucleus collisions [2]. A quantitative interpretation of the lepton-pair data can be obtained within a scenario, where the effective vector-meson masses are reduced in a hadronic environment [3, 4, 5, 6]. On the other hand, attempts to interpret the low-mass enhancement of lepton pairs in terms of many-body effects also yield good agreement with the data [7, 8]. In these calculations the broadening of the ρ meson in nuclear matter due to the interactions of its pion cloud with the medium [9, 10, 11, 12] and the momentum dependence of the ρ-meson self energy due to the coupling with baryon-resonance–nucleon-hole states [13, 14] are taken into account.