Measurements of $ep \to e^\prime \pi^+n$ at W = 1.6 - 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS

TL;DR

This paper reports precise measurements of the $ep o e' ext{π}^+ n$ cross sections over a range of energies and momentum transfers, extracting nucleon resonance electrocouplings and comparing them with theoretical models.

Contribution

It provides new high-precision data on nucleon resonance electroexcitation amplitudes and tests quark model predictions using advanced analysis methods.

Findings

Significant $N(1675)5/2^-$ strength in $A_{1/2}$ amplitude contradicts quark model predictions.

Observation of a transition in dominance from $A_{3/2}$ to $A_{1/2}$ for $N(1680)5/2^+$ with increasing $Q^2$.

Rapid decline of $S_{1/2}$ amplitude with $Q^2$ aligns with quark model expectations.

Abstract

Differential cross sections of the exclusive process $e p \to e^\prime \pi^+ n$ were measured with good precision in the range of the photon virtuality $Q^2 = 1.8 - 4.5$ GeV$^2$, and the invariant mass range of the $\pi^+ n$ final state W = 1.6 - 2.0 GeV using the CEBAF Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the $n\pi^+$ center-of-mass system. More than 37,000 cross section points were measured. The contributions of the isospin $I = {1\over 2}$ resonances $N(1675){5\over 2}^-$, $N(1680){5\over 2}^+$ and $N(1710){1\over 2}^+$ were extracted at different values of $Q^2$ using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-$t$ dispersion relations, were employed in the analysis. We observe significant strength of the $N(1675){5\over 2}^-$ in the $A_{1/2}$ amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the $N(1680){5\over 2}^+$ we observe a slow changeover from the dominance of the $A_{3/2}$ amplitude at the real photon point ($Q^2=0$) to a $Q^2$ where $A_{1/2}$ begins to dominate. The scalar amplitude $S_{1/2}$ drops rapidly with $Q^2$ consistent with quark model prediction. For the $N(1710){1\over 2}^+$ resonance our analysis shows significant strength for the $A_{1/2}$ amplitude at $Q^2 < 2.5$ GeV$^2$.