A covariant model for the negative parity resonances of the nucleon

Abstract

We present a model for the $\gamma^\ast N \to N^\ast$ helicity amplitudes, where $N$ is the nucleon and $N^\ast$ is a negative parity nucleon excitation, member of the $SU(6)$-multiplet $[70,1^-]$. The model combines the results from the single quark transition model for the helicity amplitudes with the results of the covariant spectator quark model for the $\gamma^\ast N \to N^\ast(1535)$ and $\gamma^\ast N \to N^\ast(1520)$ transitions. With the knowledge of the amplitudes $A_{1/2}$ and $A_{3/2}$ for those transitions we calculate three independent coefficients defined by the single quark transition model and make predictions for the helicity amplitudes associated with the $\gamma^\ast N \to N^\ast(1650)$, $\gamma^\ast N \to N^\ast(1700)$, $\gamma^\ast N \to \Delta(1620)$, and $\gamma^\ast N \to \Delta(1700)$ transitions. In order to facilitate the comparison with future experimental data at high $Q^2$, we provide also simple parametrizations for the amplitudes, compatible with the expected falloff at high $Q^2$.