Measurement of Spin-Density Matrix Elements in $\Delta^{++}(1232)$ photoproduction

TL;DR

This paper reports precise measurements of the spin-density matrix elements in $^{++}(1232)$ photoproduction, revealing the significant role of unnatural exchange processes at low momentum transfer.

Contribution

It provides the first high-precision SDME measurements in this reaction, clarifies the relative sign of couplings in Regge models, and decomposes exchange amplitudes.

Findings

Unnatural exchange is significant at low momentum transfer.

The measurement exceeds previous precision by three orders of magnitude.

Data constrains Regge-exchange model parameters.

Abstract

We measure the spin-density matrix elements (SDMEs) of the $\Delta^{++}(1232)$ in the photoproduction reaction $\gamma p \to \pi^-\Delta^{++}(1232)$ 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 $1.4$ GeV$^2$. 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.