Electroexcitation of the Roper resonance from CLAS data

TL;DR

This study analyzes the electroexcitation of the Roper resonance using CLAS data, revealing the Q2-dependent behavior of helicity amplitudes and supporting its interpretation as a radial excitation rather than a hybrid state.

Contribution

It provides the first detailed extraction of helicity amplitudes for the Roper resonance over a range of Q2 using two consistent approaches, clarifying its internal structure.

Findings

Transverse helicity amplitude changes sign around Q2=2 GeV2.

Longitudinal amplitude decreases with increasing Q2.

Results support Roper as a radial excitation, not a hybrid state.

Abstract

The helicity amplitudes of the electroexcitation of the Roper resonance on proton are extracted at 1.7 < Q2 < 4.2 GeV2 from recent high precision JLab-CLAS cross sections data and longitudinally polarized beam asymmetry for pi+ electroproduction on protons. The analysis is made using two approaches, dispersion relations and unitary isobar model, which give consistent results. It is found that the transverse helicity amplitude for the gamma* p --> P11(1440) transition, which is large and negative at Q2=0, becomes large and positive at Q2 ~ 2 GeV2, and then drops slowly with Q2. Longitudinal helicity amplitude, that was previously found from CLAS data as large and positive at Q2=0.4,0.65 GeV2, drops with Q2. These results rule out the presentation of P11(1440) as a 3qG hybrid state, and provide strong evidence in favor of this resonance as a first radial excitation of the 3q ground state.