Nuclear-medium modification of the ρ0(1S) and ρ′(2S) mesons in coherent photo-and electroproduction: Coupled-channel analysis

We study medium modifications of the dilepton e+e− and μ+μ− mass spectra in coherent photo-and electroproduction of ρ0(1S)-and ρ′(2S)-meson resonances on nuclear targets. The analysis is performed within the coupled ρ0(1S), ρ′(2S), ... channel formalism, where nuclear modifications derive from off-diagonal rescatterings. We find that the effect of off-diagonal rescatterings on the shape of the dilepton-mass spectrum in the ρ0(1S)-meson mass region is only marginal, but it is very important in the ρ′(2S) mass region. The main off-diagonal contribution in the ρ′(2S) mass region comes from the sequential mechanism γ* → ρ0(1S) → ρ′(2S), which dominates ρ′(2S) production for heavy nuclei. Our results also show that, in the ρ′(2S) mass region, there is a considerable interference of the Breit-Wigner tail of the amplitude for the decay ρ0(1S) to e+e− and μ+μ− with the amplitude for the decay of ρ′(2S) to e+e− and μ+μ−.