Extraction of electromagnetic transition form factors for nucleon resonances within a dynamical coupled-channels model

Abstract

We explain the application of a recently developed analytic continuation method to extract the electromagnetic transition form factors for the nucleon resonances (N * ) within a dynamical coupled-channel model of mesonbaryon reactions. Illustrative results of the obtained N * → γ N transition form factors, defined at the resonance pole positions on the complex energy plane, for the well-isolated P 33 and D 13 and the complicated P 11 resonances are presented. A formula was developed to give a unified representation of the effects due to the first two P 11 poles, which are near the π threshold, but are on different Riemann sheets. We also find that a simple formula, with its parameters determined in the Laurent expansions of the πN → πN and γ N → πN amplitudes, can reproduce to a very large extent the exact solutions of the considered model at energies near the real parts of the extracted resonance positions. We discuss the important differences between our approach, which is consistent with the earlier formulations of resonances, and the phenomenological approaches using the Breit-Wigner parametrization of resonant amplitudes to fit the data.

Experimental study of theP11(1440)andD13(1520)Mokeev1, Burkert2, Elouadrhiri3 et al. 2012Phys. Rev. C13231680View full textAdd to dashboardCite

“…The DSEQCD results are close to the bare-quark-core contributions to the P 11 (1440) electrocouplings extracted from the experimental data within the framework of the EBAC-DCC coupled-channel approach [23] and far from the dressed P 11 (1440) electrocouplings obtained in our analysis. This DSEQCD result offers further evidence for substantial contributions from meson-baryon dressing to the P 11 (1440) electrocouplings.…”

“…The N * parameters extracted from the data in this way can only be compared directly with those obtained from the same or other exclusive channel data fits that employ BW parametrizations of resonant amplitudes. It remains to be seen how these resonance parameters can be related to the values determined from singularities of the reaction amplitudes in the complex energy plane, as employed in coupled-channel analyses [23]. In the Section II C we described the JM model relations between the resonance parameters and the model independent observables: the resonant part of the cross sections and N * electromagnetic/hadronic decay widths.…”

“…The global analysis of N π, photo-, electro-, and hadroproduction within the framework of the EBAC-DCC coupled-channel approach [17,18,23,[70][71][72][73] revealed this additional component in the N * structure that is most relevant at Q 2 < 1.0 GeV 2 . The general unitarity condition for full meson electroproduction amplitudes requires contributions from non-resonant meson electroproduction and hadronic scattering amplitudes to the γ v pN * vertex, as depicted in Fig.…”

“…To the best of our knowledge, there are no models, incorporating only quark degrees of freedom, capable of describing simultaneously the mass ordering of the P 11 (1440) and S 11 (1535) resonances, the unusually large P 11 (1440) hadronic decay width, and the photo and electrocouplings. Therefore, different approaches for the description of the P 11 (1440) were developed that take into account the contributions from degrees of freedom other than those of dressed quarks [17,23,63,76]. The CLAS results on the P 11 (1440) electrocouplings of [15] and this analysis allows for the first time to confront the model expectations for the P 11 (1440) structure with the experimental data, in particular at Q 2 < 0.6 GeV 2 , where consistent results from the independent analyses of N π and π + π − p electroproduction off protons have now become available.…”

“…Studies of meson-baryon dressing contributions to γ v pN * electrocouplings and resonance hadronic decay amplitudes, carried out at the Excited Baryon Analysis Center (EBAC) at Jefferson Lab [17][18][19][20][21][22][23][24], have extended our insight into the spectrum and structure of excited nucleon states considerably. The contributions from meson-baryon dressing to the P 11 (1440) and D 13 (1520) electrocouplings were determined from a global analysis of the world data on πN scattering and π + n, π 0 p electroproduction off protons within the framework of the EBAC dynamical coupledchannel approach (EBAC-DCC) [23].…”