Target and Beam-Target Spin Asymmetries in Exclusive Pion Electroproduction for Q\u3csup\u3e2\u3c/sup\u3e \u3e 1 GeV\u3csup\u3e2\u3c/sup\u3e . I. ep → eπ\u3csup\u3e+\u3c/sup\u3en

Abstract

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π+ electroproduction reaction γ∗p→nπ+. 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 \u3c W \u3c 3 GeV and 1 \u3c Q2 \u3c 6 GeV2. Results were obtained for about 6000 bins in W, Q2, 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 \u3c 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 Q2, for resonances with masses as high as 2.4 GeV. © 2017 American Physical Society