Electromagnetic Transition Form Factors of Nucleon Resonances

TL;DR

This paper discusses recent CLAS measurements of nucleon resonance transition form factors, resolving longstanding puzzles, confirming quark model predictions, and providing new insights into resonance behaviors at various momentum transfers.

Contribution

It presents new experimental data that clarify the nature of the Roper resonance and other nucleon resonances, supporting the symmetric constituent quark model and detailing transition amplitude behaviors.

Findings

Roper resonance confirmed as the first radial excitation of the nucleon.

Slow fall off of the $S_{11}(1535)$ transition form factor with $Q^2$.

Rapid change in $D_{13}(1520)$ resonance helicity dominance with $Q^2$.

Abstract

Recent measurements of nucleon resonance transition form factors with CLAS at Jefferson Lab are discussed. The new data resolve a long-standing puzzle of the nature of the Roper resonance, and confirm the assertion of the symmetric constituent quark model of the Roper as the first radial excitation of the nucleon. The data on $n\pi^+$ production at high $Q^2$ confirm the slow fall off of the $S_{11}(1535)$ transition form factor with $Q^2$, and better constrain the branching ratios $\beta_{N\pi}=0.50$ and $\beta_{N\eta}=0.45$. Also, new results on the transition amplitudes for the $D_{13}(1520)$ resonance are presented showing a rapid change from helicity 3/2 dominance seen at the real photon point to helicty 1/2 dominance at higher $Q^2$.