Pion electro-production in the Roper region in chiral quark models

TL;DR

This paper develops a coupled-channel method to calculate pion electro-production amplitudes in the Roper resonance region, successfully accounting for resonance mixing and decay channels, and demonstrates its effectiveness within a chiral quark model.

Contribution

It introduces a novel coupled-channel approach to extract resonant amplitudes in pion electro-production, incorporating quasi-bound quark-model states and pion cloud effects.

Findings

Good agreement with observed M1- amplitude, highlighting pion cloud contributions.

Significant pion cloud effects observed in S1- amplitude, though data uncertainties remain.

Method effectively isolates resonant parts despite channel mixing.

Abstract

We present a method to calculate pion electro-production amplitudes in a coupled-channel framework incorporating quasi-bound quark-model states. The method offers a clear prescription how to extract the resonant part of the amplitudes, even in the presence of different decay channels and a strong mixing of neighbouring resonances. The method is applied to the calculation of the M1- and the S1- amplitudes in the P11 partial wave in a simple chiral quark model. A good agreement with the observed M1- amplitude is found with a significant contribution from the pion cloud. The same effect is also prominent in the S1- amplitude but a rather uncertain data prevent us to draw a definitive conclusion.