Measurement of Spin-Density Matrix Elements in Δ⁺⁺(1232) photoproduction

We report the measurement of spin-density matrix elements of the Δ⁺⁺(1232) in the photoproduction reaction γ p → π⁻Δ⁺⁺(1232) with the GlueX experiment in Hall D at Jefferson Lab. The measurement used a linearly polarized photon beam with E_γ=8.2-8.8~GeV and the statistical precision exceeds the previous measurement from SLAC by three orders of magnitude for the momentum transfer squared region $-t < 1.4$ GeV². The data are sensitive to the previously undetermined relative sign between couplings in existing Regge exchange models. Linear combinations of the extracted SDMEs allow for a decomposition into natural and unnatural exchange amplitudes, which shows that the unnatural exchange plays an important role in the low $-t$ region.