Electroexcitation of the Roper resonance for 1.7 < Q2 < 4.5 GeV2 in ep -> enpi+

TL;DR

This paper analyzes the electroexcitation helicity amplitudes of the Roper resonance at intermediate Q2, using high-precision data and two theoretical approaches, providing insights into its structure and nature.

Contribution

It presents the first extraction of Roper resonance helicity amplitudes at 1.7-4.5 GeV2 Q2 using dispersion relations and isobar models, clarifying its internal structure.

Findings

A_{1/2} amplitude changes from negative to positive around Q2=2 GeV2.

S_{1/2} amplitude decreases with increasing Q2.

Results support N(1440)P11 as a radial excitation of the 3-quark ground state.

Abstract

The helicity amplitudes of the electroexcitation of the Roper resonance are extracted for 1.7 < Q2 < 4.5 GeV2 from recent high precision JLab-CLAS cross section and longitudinally polarized beam asymmetry data for pi+ electroproduction on protons at W=1.15-1.69 GeV. The analysis is made using two approaches, dispersion relations and a unitary isobar model, which give consistent results. It is found that the transverse helicity amplitude A_{1/2} for the gamma* p -> N(1440)P11 transition, which is large and negative at Q2=0, becomes large and positive at Q2 ~ 2 GeV2, and then drops slowly with Q2. The longitudinal helicity amplitude S_{1/2}, which was previously found from CLAS ep -> eppi0,enpi+ data to be large and positive at Q2=0.4,0.65 GeV2, drops with Q2. Available model predictions for gamma* p -> N(1440)P11 allow us to conclude that these results provide strong evidence in favor of N(1440)P11 as a first radial excitation of the 3q ground state. The results of the present paper also confirm the conclusion of our previous analysis for Q2 < 1 GeV2 that the presentation of N(1440)P11 as a 3qG hybrid state is ruled out.