The role of the $qqqq\bar{q}$ components in the electromagnetic transition $\gamma^*N\to N^*(1535)$

TL;DR

This study extends the quark model to include 5-quark components to better explain the electromagnetic transition amplitudes of the N*(1535) resonance, achieving partial agreement with experimental data.

Contribution

The paper introduces a quark model with 5-quark components to improve the description of N*(1535) electromagnetic transitions, highlighting the role of $qqqqar{q}$ admixtures.

Findings

Including 5-quark components reduces the helicity amplitude $A^{p}_{1/2}$ at $Q^2=0$ to match empirical data.

$qqqqar{q}$ components with $sar s$ pairs explain the $N^{*}(1535)$ decay width.

More compact 5-quark components better fit the momentum dependence of amplitudes.

Abstract

The helicity amplitudes $A^{p}_{1/2}$ and $S^{p}_{1/2}$ for the electromagnetic transition $\gamma^{*} N\to N^{*}(1535)$ are calculated in the quark model that is extended to include the lowest lying $qqqq\bar{q}$ components in addition to the $qqq$ component. It is found that with admixture of 5-quark components with a proportion of 20% in the nucleon and 25-65% in the $N^{*}(1535)$ resonance the calculated helicity amplitude $A^{p}_{1/2}$ decreases at the photon point, $Q^{2}=0$ to the empirical range. The $qqqq\bar{q}$ components contain $s\bar s$ pairs, which is consistent with the substantial width for $N\eta$ decay of the $N^{*}(1535)$. The best description of the momentum dependence of the empirical helicity amplitudes is obtained by assuming that the $qqqq\bar{q}$ components are more compact than the $qqq$ component. However, this version of extended quark model still does not lead to a satisfactory simultaneous description of both $A^{p}_{1/2}$ and $S^{p}_{1/2}$ although with significant improvement.