γn(p)→π⁻p(p) Asymmetries with Linearly Polarized Beams and Longitudinally Polarized Targets in the N^∗ Resonance Region

The excited-state spectrum of the nucleon is a complicated overlap of many resonances that must be disentangled through multipole analyses of reaction amplitudes. Meson photoproduction, which has been a fruitful probe of N^∗ structure, requires data on many different polarization observables to constrain its four complex amplitudes. While considerable data has been accumulated with proton targets, comparatively little information is available from neutron targets. Recently, the first beam-target helicity asymmetries with circular beam polarization in the γn(p)→π⁻p(p) reaction have been reported (Hao et al. in PRL 118:242002, 2017). This paper presents a parallel analysis from the same experiment of polarization observables with linearly polarized beams for the same reaction. Linearly polarized photons and longitudinally polarized deuterons in a solid hydrogen deuteride target were used with the CEBAF Large Acceptance Spectrometer at Jefferson lab (JLab). Data are combined to extract the beam (Σ) and beam-target double-polarization (G) asymmetries. Preliminary results for the Σ observables are consistent with existing partial wave analyses (PWA) that incorporate other experiments. Preliminary results for the energy and angular dependence of G are reported; these deviate strongly from existing PWA. (author)