Electromagnetic Transition Form Factors and Dilepton Decay Rates of Nucleon Resonances

Abstract

Relativistic, kinematically complete phenomenological expressions for the dilepton decay rates of nucleon resonances with arbitrary spin and parity are derived in terms of the magnetic, electric, and Coulomb transition form factors. The dilepton decay rates of the nucleon resonances with masses below 2 GeV are estimated using the extended vector meson dominance (VMD) model for the transition form factors. The model provides a unified description of the photo-and electroproduction data, γ (γ * )N → N * , the vector meson decays, N * → N ϕ (ω), and the dilepton decays, N * → N + − . The constraints on the transition form factors from the quark counting rules are taken into account. The parameters of the model are fixed by fitting the available photo-and electroproduction data and using results of the multichannel partial-wave analysis of the π N scattering. Where experimental data are not available, predictions of the non-relativistic quark models are used as an input. The vector meson coupling constants of the magnetic, electric, and Coulomb types are determined. The dilepton widths and the dilepton spectra from decays of nucleon resonances with masses below 2 GeV are calculated. C 2002 Elsevier Science (USA)

Inclusive dielectron production in proton-proton collisions at 2.2 GeV beam energy

“…[21] and that of the (1232) as calculated in Ref. [1]. At the present bombarding energy the production of π 0 and η mesons is known to proceed mostly via resonance excitation (e.g., R = (1232), N * (1440), N * (1520), N * (1535), etc.)…”

“…5. Clearly additional dielectron sources are needed, among which one has to consider the decays of N * resonances, e.g., the N * (1520) and N * (1720) known to couple strongly to the ρ, as well as a possible general enhancement due to vector meson dominance form factors of the nucleon resonances [1]. The effect of adding such contributions has recently been investigated by the authors of Ref.…”

“…Dilepton (that is e + e − or μ + μ − ) observables indeed provide information on the electromagnetic structure of the resonances and, in the context of vector meson dominance, their coupling to the light vector mesons [1]. Furthermore, dilepton spectroscopy allows the study of the properties of hadrons produced and decayed in a strongly interacting medium.…”

“…Inclusive e + e − production in p + p and p + d reactions has formerly been studied in the range of [1][2][3][4][5] GeV by the dilepton spectrometer (DLS) experiment at the Bevalac facility [13] and has more recently been studied by the HADES experiment at 1.25 [7] and 3.5 GeV [8]. In particular, the comparison of the latter data sets with various model calculations demonstrated a need for improved theoretical descriptions.…”

Resonance production and decay in pion induced collisions with HADES

“…Another approach is the Vector Meson Dominance (VMD) model, introduced by J.J. Sakurai [7], that assumes that the electromagnetic current is mediated by ground state vector mesons, which yields monopole form factors; however it was proved that the standard VMD model overestimates the branching ratios of the nucleon resonances for radiative decay. To circumvent this problem, extended VMD [8], in which the excited states of the vector mesons ρ , ρ , ... are taken into account for the description of the transition form factors of the nucleon resonances, was introduced. Constituent quark models for transition form factors used for the space like region have also been extended recently to the time like region [9], [10].…”