Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

TL;DR

This paper analyzes CLAS data on pion electroproduction to study nucleon resonances, revealing insights into meson-cloud contributions, resonance structures, and electrocoupling amplitudes across a range of Q2 values.

Contribution

It provides a comprehensive analysis of nucleon resonance electroexcitation using two different models, offering new insights into resonance structures and model sensitivities.

Findings

Meson-cloud contributions are crucial for Delta(1232)P33 magnetic strength.

N(1440)P11 is confirmed as a radial excitation of a 3-quark state.

Electrocouplings for N(1520)D13 and N(1535)S11 are extracted with high precision.

Abstract

We present results on the electroexcitation of the low mass resonances Delta(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11 in a wide range of Q2. The results were obtained in the comprehensive analysis of JLab-CLAS data on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for pion electroproduction off the proton. The data were analysed using two conceptually different approaches, fixed-t dispersion relations and a unitary isobar model, allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Delta(1232)P33} show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q2<6 GeV2. For the Roper resonance, N(1440)P11, the data provide strong evidence for this state as a predominantly radial excitation of a 3-quark ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S11 allowed us to obtain the branching ratios of this state to the piN and etaN channels via comparison to the results extracted from eta electroproduction. The extensive CLAS data also enabled the extraction of the gamma*p -> N(1520)D13 and N(1535)S11 longitudinal helicity amplitudes with good precision.