Cross sections and beam asymmetries for $\vev{e}p \to en\pi^+$ in the nucleon resonance region for $1.7 \le Q^2 \le 4.5 (GeV)^2$

TL;DR

This study measures exclusive electron-proton scattering producing a neutron and pi-plus in the nucleon resonance region, providing new data on structure functions and asymmetries across a wide kinematic range to improve understanding of nucleon excitations.

Contribution

First measurement of $e p o e' n \pi^+$ in specific kinematics with nearly full angular coverage, offering new insights into nucleon resonance transition form factors.

Findings

Discrepancy observed in the $\sigma_{LT'}$ structure function.

Measured over 31,000 cross sections and 4,000 asymmetries.

Data enables model-independent analysis of nucleon resonance excitations.

Abstract

The exclusive electroproduction process $\vec{e}p \to e^\prime n \pi^+$ was measured in the range of the photon virtuality $Q^2 = 1.7 - 4.5 \rm{GeV^2}$, and the invariant mass range for the $n\pi^+$ system of $W = 1.15 - 1.7 \rm{GeV}$ using the CEBAF Large Acceptance Spectrometer. For the first time, these kinematics are probed in exclusive $\pi^+$ production from protons with nearly full coverage in the azimuthal and polar angles of the $n\pi^+$ center-of-mass system. The $n\pi^+$ channel has particular sensitivity to the isospin 1/2 excited nucleon states, and together with the $p\pi^0$ final state will serve to determine the transition form factors of a large number of resonances. The largest discrepancy between these results and present modes was seen in the $\sigma_{LT'}$ structure function. In this experiment, 31,295 cross section and 4,184 asymmetry data points were measured. Because of the large volume of data, only a reduced set of structure functions and Legendre polynomial moments can be presented that are obtained in model-independent fits to the differential cross sections.