First measurement of massive virtual photon emission from N^ baryon resonances

This is the authors' abstract. We don't add key points for this paper.

First information on the timelike electromagnetic structure of baryons in the second resonance region has been obtained from measurements of the dielectron (e⁺e⁻) invariant mass and angular distributions with the High Acceptance Di-Electron Spectrometer (HADES) at GSI, using a secondary pion beam impinging on CH₂ and C targets. The quasi-free reaction π⁻ + p → ne⁺e⁻ at √sπ p = 1.49 GeV is isolated in several analysis steps. The data set provides a crucial test for the description of baryon timelike transitions. Close to the kinematical limit the meassured e⁺e⁻ mass distribution shows deviations by up to a factor 8 from the Dalitz decay of a point-like baryon. Various theoretical approaches connecting information from hadronic and electromagnetic channels in the spacelike and timelike regime compare well with the measured e⁺e⁻ invariant mass distribution, including (i) a Vector Meson Dominance amplitude containing direct photon and vector meson (ρ) couplings to the baryon, (ii) electromagnetic timelike baryon transition form factors in a covariant spectator-quark model, where meson cloud effects dominate, and (iii) application of dispersion theory, which demonstrates the importance of the pion electromagnetic form factors. The dielectron angular distributions exhibit contributions of virtual photons (γ*) with longitudinal polarization, in contrast to real photons. The γ* angular dependence supports the dominance of J=3/2, I=1/2 contributions in both the γ*n and the ππN channels.