What is the structure of the Roper resonance?

Abstract

We investigate the structure of the nucleon resonance N*(1440) ͑Roper͒ within a coupled-channel meson exchange model for pion-nucleon scattering. The coupling to N states is realized effectively by the coupling to the N, ⌬, and N channels. The interaction within and between these channels is derived from an effective Lagrangian based on a chirally symmetric Lagrangian, which is supplemented by well known terms for the coupling of the ⌬ isobar, the meson, and the '','' which is the name given here to the strong correlation of two pions in the scalar-isoscalar channel. In this model the Roper resonance can be described by meson-baryon dynamics alone; no genuine N*(1440) ͑three quark͒ resonance is needed in order to fit N phase shifts and inelasticities.

Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

“…The importance of pion (cloud) contributions to the transition form factors has also been confirmed by the lattice calculations [29]. Alternative descriptions include the representation of N (1440)P 11 as a gluonic baryon excitation [30,31] and the possibility that nucleon resonances are meson-baryon molecules generated in chiral coupled-channel dynamics [32][33][34][35][36]. Relations between baryon electromagnetic form factors and generalized parton distributions (GPDs) have also been formulated that connect these two different notions to describe the baryon structure [37,38].…”

“…They cover the first, second, and part of the third resonance regions. The stages of our analysis are dictated by how we evaluate the influence of higher resonances on the extracted amplitudes for the [8] (1232)P 33 and for the resonances from the second resonance region.…”

“…This 055203-3 allowed us to get information on the electroexcitation amplitudes for the resonances from the third resonance region. This information, combined with the γp → N * amplitudes known from photoproduction data [20], sets the ranges of the higher resonance contributions when we extract the amplitudes of the γ * p → (1232)P 33 , N(1440)P 11 , N(1520)D 13 , and N(1535)S 11 transitions from the data reported in Table I. In the next step, we analyze the data from Table II, which present a large body of ep → enπ + differential cross sections and longitudinally polarized electron beam asymmetries at large Q 2 = 1.72-4.16 GeV 2 [7].…”

“…In recent years, a variety of measurements of single pion electroproduction on protons, including polarization measurements, have been performed at CLAS in a wide range of photon virtuality Q 2 from 0.16 to 6 GeV 2 [1][2][3][4][5][6][7][8]. In this work, we present the results on the electroexcitation of the resonances (1232)P 33 , N(1440)P 11 , N(1520)D 13 obtained from the comprehensive analysis of these data.…”

“…All these uncertainties are included in the total model uncertainty of the final results. So, in addition to the uncertainties in the data, we have accounted for, as much as possible, the model uncertainties of the extracted γ * N → (1232)P 33 , N(1440)P 11 , N(1520)D 13 , and N(1535)S 11 amplitudes. The results are presented in Sec.…”