Measurement of Target and Double-spin Asymmetries for the $\vec e\vec p\to e\pi^+ (n)$ Reaction in the Nucleon Resonance Region at Low $Q^2$

TL;DR

This paper presents measurements of target and double-spin asymmetries in exclusive pion production from polarized protons at low momentum transfer, extending previous data and providing constraints for theoretical models of nucleon resonances.

Contribution

It provides new low $Q^2$ asymmetry data for the $ar ear p o ear mbda^+ (n)$ reaction, utilizing a custom Cherenkov detector to access previously unexplored kinematic regions.

Findings

Results are consistent with previous measurements but extend to lower $Q^2$.

Data help constrain polarization-dependent components of nucleon resonance models.

Comparison with models shows areas of agreement and discrepancy in the resonance region.

Abstract

We report measurements of target- and double-spin asymmetries for the exclusive channel $\vec e\vec p\to e\pi^+ (n)$ in the nucleon resonance region at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). These asymmetries were extracted from data obtained using a longitudinally polarized NH$_3$ target and a longitudinally polarized electron beam with energies 1.1, 1.3, 2.0, 2.3 and 3.0 GeV. The new results are consistent with previous CLAS publications but are extended to a low $Q^2$ range from $0.0065$ to $0.35$ (GeV$/c$)$^2$. The $Q^2$ access was made possible by a custom-built Cherenkov detector that allowed the detection of electrons for scattering angles as low as $6^\circ$. These results are compared with the unitary isobar models JANR and MAID, the partial-wave analysis prediction from SAID and the dynamic model DMT. In many kinematic regions our results, in particular results on the target asymmetry, help to constrain the polarization-dependent components of these models.