Measurement of spin-density matrix elements in Δ++(1232) photoproduction

Abstract

We measure the spin-density matrix elements (SDMEs) of the ${\Delta }^{++}\left(1232\right)$ in the photoproduction reaction $\gamma p\to {\pi }^{-}{\Delta }^{++}\left(1232\right)$ with the GlueX experiment in Hall D at Jefferson Lab. The measurement uses a linearly–polarized photon beam with energies from 8.2 to 8.8 GeV and the statistical precision of the SDMEs exceeds the previous measurement by three orders of magnitude for the momentum transfer squared region below 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. We find that the unnatural exchange plays an important role in the low momentum transfer region.