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 $Q^2$.

Comments: Proceedings of the workshop "XIII International Workshop on Hadron Physics", 22-27 March 2015, Hotel do Bosque, Angra dos Reis - RJ, Brazil. 8 pages, 7 figures