Jefferson Lab’s Positron Working Group advocates for adding a positron source to the CEBAF accelerator, which would allow a whole host of possible new positron-scattering experiments. This past year, the group has produced a white paper which has now been peer reviewed and published as a topical issue of the European Physical Journal A. The GW team contributed to six of the issue’s papers, all relating to using differences between electron scattering and positron scattering to quantify the effects of two-photon exchange. One paper was an all-GW affair: “Target-normal single spin asymmetries measured with positrons” by Gabe, Tyler, and Axel (EPJA 57:213, 2021). In this study, we argue that a positron beam, combined with the new Super-Big-Bite Spectrometer, and a transversely polarized proton target, would allow a first-ever measurement of two photon exchange through a quantity called the “target-normal single spin asymmetry,” labeled by An in the figure below.
This was Gabe’s first publication. He developed and ran the code to determine the theoretical predictions of the GPD model developed by GW’s Andrei Afanasev, and went on to produce all of the paper’s figures, including the one above.
With the Electron Ion Collider being built at Brookhaven, the future of Jefferson Lab is open to a number of possibilities. We see great reasons for that future to include a unique, world-leading positron scattering facility.