Quark model calculations of transition form factors at high photon virtualities

Abstract

We present calculations of $\gamma^\ast N \to N^\ast$ transition form factors, where $N$ is the nucleon and $N^\ast$ is a nucleon resonance, based on a covariant quark model. Our main focus is at high photon virtualities (large $Q^2$) where the valence quark degrees of freedom dominate the contributions to the transition form factors and helicity amplitudes. In that regime, the quark model estimates can be compared with the available data, particularly with the Jefferson Lab data at intermediate and large momentum transfer ($Q^2 >2$ GeV$^2$). The main focus is on the $\Delta(1232)3/2^+$, $N(1440)1/2^+$, $N(1535)1/2^-$ and $N(1520)3/2^-$ resonances, but estimates for other higher mass resonances are also discussed.