Target and beam-target spin asymmetries in exclusive pion electroproduction for ${Q}^{2}&gt;1\phantom{\rule{0.16em}{0ex}}{\mathrm{GeV}}^{2}$. I. $ep\ensuremath{\rightarrow}e{\ensuremath{\pi}}^{+}n$

Target and beam-target spin asymmetries in exclusive pion electroproduction for $Q^{2} > 1 {GeV}^{2}$ . I. $e p \to e π^{+} n$

P. E. Bosted et al. (CLAS Collaboration)

Phys. Rev. C 95, 035206 – Published 20 March 2017

Abstract

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive $π^{+}$ electroproduction reaction $γ^{*} p \to n π^{+}$ . The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic range covered is $1.1 < W < 3$ GeV and $1 < Q^{2} < 6 {GeV}^{2}$ . Results were obtained for about 6000 bins in $W, Q^{2}, cos (θ^{*})$ , and $ϕ^{*}$ . Except at forward angles, very large target-spin asymmetries are observed over the entire $W$ region. Reasonable agreement is found with phenomenological fits to previous data for $W < 1.6$ GeV, but very large differences are seen at higher values of $W$ . A generalized parton distributions (GPD)-based model is in poor agreement with the data. When combined with cross-sectional measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^{2}$ , for resonances with masses as high as 2.4 GeV.