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The annual in-person meeting of the Jefferson Lab Positron Working Group was held at George Washington University over March 18–20, 2024. Axel served as the local organizer, working with Eric Voutier (IJC Lab, Orsay, France), Douglas Higinbotham (JLab), and Xiaochao Zheng (UVa). The meeting included talks on the engineering challenges with producing and accelerating a positron beam in Jefferson Lab’s CEBAF accelerator, ideas for new experiments that could be conducted with positrons, and updates on theory related to two photon exchange, generalized parton distributions, and other topics that positrons could hopefully address. Since the last meeting, the Jefferson Lab positron program added five conditionally approved experiments in a highly successful showing at the 2023 JLab PAC. This year, our discussions revolved around advancing R&D efforts to make positrons a reality.

In a stroke of good luck, the meeting coincided with the peak of the cherry blossom season in Washington, and we enjoyed an excellent afternoon excursion to the tidal basin where we were able to pay our respects to Stumpy.

This past November, the American Physical Society’s Division of Nuclear Physics held its fall meeting in conjunction with the Physical Society of Japan on the Big Island of Hawaii. Prof. Strakovsky organized a workshop as part of the conference, titled “Spectroscopy of Hyperons and Heavy Baryons at JLab and J-PARC,” and spoke about the future K-Long Facility at Jefferson Lab. He also presented our work looking for the Pc(4312)+ pentaquark in data from GlueX. Undergraduate student, Quinn Stefan, participated in the “Conference Experience for Undergraduates” (CEU) program, and presented a poster (shown above) about her work on Radiative Corrections.

The CaFe Experiment, studying short range correlations in Calcium (Ca) and Iron (Fe) is underway in Hall C at Jefferson Lab. We use the High Momentum Spectrometer (HMS) and the Super High Momentum Spectrometer (SHMS) to detect protons and electrons, respectively, emerging from collisions with the target nuclei.

A few of our group’s projects extend beyond Jefferson Lab. One such project is the MUSE Experiment, being conducted at the Paul Scherrer Institute (PSI) in Switzerland. The Experiment is lead by GW’s Prof. Downie, and will attempt to resolve the proton radius puzzle by measuring the proton radius twice; once with electrons and once with unstable muon particles.

Bill and Peter have contributed to a recent paper, published in the journal Physical Review C, documenting studies of the MUSE beam-line. The incoming particles in MUSE are actually a secondary beam, produced by primary collisions of protons on a production target. The MUSE beam-line transports muons, pions, electrons and positrons to the MUSE proton target. It is crucial for the experiment that the muon and electron beams have identical properties.

Our group’s new analysis on nucleon sum rules has now been published in the journal Physical Review C. The Gerasimov-Drell-Hearn (GDH) sum rule, the Baldin sum rule, and the Gell-Mann–Goldberger–Thirring (GGT) sum rule are important relationships between photo-production cross sections and fundamental properties of protons and neutrons: their anomalous magnetic moments, their electromagnetic polarizabilities, and forward spin polarizability, respectively. While these sum rules cover the probabilities for all possible combinations of particles produced in photon scattering, in principle the most important contribution comes from the reaction in which one pion is produced.

To study the single-pion contribution, we looked at predictions of SAID, a global partial wave analysis tool developed at GW. One of the striking findings is that the convergence of the GDH sum rule is very different for protons and neutrons. Whereas the GDH sum rule is nearly entirely satisfied by single-pion production for protons, on the neutron, single pion production only satisfies about 60% of the sum rule.

Still to tackle is the Schwinger sum rule, which involves a particular contribution to the electro-production cross section called the LT interference. We plan to address the convergence of this sum rule in a future paper.

On Nov. 23rd, Chan Kim successfully defended his PhD thesis, titled “Measurement of the Helicity Asymmetry E for the γp → π0p reaction in the Resonance Region.” Chan analyzed data from the “FROST” experiment at Jefferson Lab, which used a polarized proton target in the form of frozen butanol beads. A polarized photon beam was scattered from the target, and Chan was interested in collisions that produced a single pi0 meson. In his analysis, Chan determined the slight difference in scattering rates when the photons and protons had their spins aligned versus anti-aligned. This can help reveal excited baryon resonances and in turn help us better understand the different ways quarks can bind together.