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

Abstract

The helicity amplitudes $A_{1/2}^p$ and $S_{1/2}^p$ for the electromagnetic transition ${\gamma }^{\ast }N\to N^{\ast }(1535)$ 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 $N^{\ast }(1535)$ resonance the calculated helicity amplitude $A_{1/2}^p$ 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 $N^{\ast }(1535)$. 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 $A_{1/2}^p$ and $S_{1/2}^p$ although with significant improvement.

Comments: version to appear in Eur. Phys. J. A