Graduate student Phoebe Sharp, and undergraduate Olivia Nippe-Jeakins were honored on October 23rd by the ARCS Foundation’s Metro Washington Chapter. In an awards ceremony hosted at the National Academy of Sciences, Phoebe was named as one of the 2025-26 MWC Chapter Scholars, and Olivia was named as the 2025-26 Toni and Hans Schierling Undergraduate Scholar. They were two of four George Washington University students honored, and of 21 MWC scholars across all member schools.
Phoebe, presenting her work on studying short-range nucleon-nucleon correlations using high-energy photon scattering, and the rho-0 photoproduction reaction in particular.
The Achievement Rewards for College Scientists (ARCS) Foundation is a national, all volunteer, women’s organization that advances science and technology in the United States by providing awards to academically outstanding US citizens studying to complete degrees in science, engineering, and medical research. The Metro Washington Chapter (MWC) of the ARCS Foundation makes annual awards to graduate and undergraduate students attending five area universities: Georgetown, George Washington, Johns Hopkins, the University of Maryland, and the University of Virginia.
Olivia presenting her work in the field of astrophysics, conducted under the mentorship of GW Prof. Alexander van der Horst, which he has undertaken in addition to her research in experimental nuclear physics.
Our latest paper, published as an editor’s suggestion in Physical Review Letters, presents the first measurement of sub-threshold photo-production of the J/ψ meson from protons bound inside nuclei. The J/ψ meson is an unstable particle formed from a charm quark bound to an anti-charm quark, and has a mass about three times that of the proton. It quickly decays, and often results in a high energy electron and positron. It takes quite a bit of energy to make J/ψ mesons, and it is just barely in reach in collisions at Jefferson Lab on stationary proton targets, i.e., the accelerator exceeds the energy threshold for the reaction. When using a nucleus as a target, J/ψ mesons can be produced even with energy beneath that threshold. This is because protons inside nuclei are moving! Their own kinetic energy within the nucleus can contribute to the reaction, and be turned into mass.
Fig. 1 from our paper, showing how the J/ψ meson can be identified as a peak in the spectrum of masses inferred from the detected electron and positron.
Why study the J/ψ? Since it’s constituent quarks are so heavy, the primary mechanism for making one in is through a reaction called gluon exchange. The gluon is the fundamental particle that mediates the strong nuclear force. The angular distribution of J/ψ mesons turns out to be related to the spatial distribution of the gluon field within the proton.
The story gets even more interesting when looking at protons bound inside nuclei. Nuclear physicists have already known for over four decades that the motion of quarks inside bound protons is distorted compared to quarks in free protons, an observation called the “EMC Effect.” We don’t know if that distortion also happens for gluons. While our measurement isn’t conclusive, there are some suggestive hints. My collaborators and I have already proposed a longer follow up experiment to Jefferson Lab to this year’s Program Advisory Committee.
Data for this paper were collected in a 2021 experiment at Jefferson Lab’s Experimental Hall D, using the GlueX spectrometer, pictured above.
Our group’s latest result is a paper published in the European Physical Journal A on radiative corrections for specific type of electron-proton scattering experiment, called a Super Rosenbluth experiment. Elastic electron-proton scattering experiments are useful for learning about how the charge and magnetism of a proton are distributed within its volume. At low electron beam energies, it makes sense for the experiment to detect the scattered electron, since very little energy is transferred to the proton, since it is comparatively heavy. At higher beam energies, there is a significant “kick” given to the proton, and experiments can be conducted in which the electron and proton are detected at the same time, that is, in coincidence. In a super-Rosenbluth experiment, only the kicked proton is detected. Proponents of this technique have claimed many benefits, one of which is the supposed reduction in complexity of radiative corrections.
To test this, we simulated traditional electron detection and super-Rosenbluth experiments using two different radiative corrections models. The first model employed the peaking approximation, an approximation that charged particles will only radiate energy in their direction of motion. The second model did not make this approximation. We found that when using the peaking approximation, radiative corrections were indeed smaller and less kinematically-dependent when performing a super-Rosenbluth experiment.
By contrast, when using a model that avoided the peaking approximation, radiative corrections were significantly larger.
Radiative tail simulated for a super-Rosenbluth experiment, using a model that avoided the peaking approximation. The tail is larger and more numerically unstable than when using the peaking approximation. The effect went away when eliminating radiation from the proton. (Figure by Quinn Stefan)
We were able identify that the larger radiative tail was caused by radiation from the proton, rather than from the electron. Specifically, if the proton were to radiate a photon in the direction of an elastically scattered electron, there could be a large enhancement in cross section.
This reaction is not modeled in the peaking approximation. For that reason, we conclude that the peaking approximation is dubious for super-Rosenbluth experiments.
Undergraduate Quinn Stefan began working on this project as part of her Luther Rice fellowship in 2023.
This past week, the American Physical Society’s annual Division of Nuclear Physics (DNP) Fall Meeting was held in Boston, MA. Six students from our group attended to present their latest work.
Izzy showed their thesis results on eta-meson electroproduction. This was their triumphant final presentation under the GW banner, as they are moving to Bochum, Germany to begin a postdoctoral position at the end of the month.
In a session on short-range nucleon nucleon correlations, Phoebe and Sara both gave updates on their thesis projects. Phoebe presented her preliminary results studying short-range correlations through rho-meson photo-production. Sara discussed her theoretical work on modeling spectator tagged structure functions in helium-4. Her first result was published this past winter.
Quinn delivered her first ever conference talk at this meeting, showing preliminary results on π- meson photoproduction from deuterium, analyzing data from the 2021 Hall D Short-Range Correlations / Color-Transparency experiment. Quinn attended last year’s DNP conference, but presented a poster.
Olivia and August presented posters at the undergraduate poster session. They both earned financial support from the Conference Experience for Undergraduates program.
Photo credit: Quinn Stefan
The end of the conference coincided with a stunning aurora over the north east, which was visible both in Boston and DC. The flight home was quite dramatic.
Axions are a hypothetical new class of particles, whose existence has been theorized as a way of solving two major open problems in physics. The first is the so-called “Strong CP Problem,” the observation that the strong nuclear force seems to be completely symmetric under charge-conjugation and parity transforms. There’s no need for this symmetry and it is an open question as to why the strong force obeys it. The second is the nature of dark matter, which has been repeatedly observed in all manner of astronomical observations, but has never been tangibly manipulated on earth. Dark matter might be made of axions, and these axions might force the strong force to be CP symmetric. It’s an attractive combo, and makes the search for axions an important pursuit.
Our experiment searched for photon pairs from the decay of axions produced through the Primakoff process.
Our group has recently published a paper in Physics Letters B in which we perform a search for axion-like particles in the data from our recent Short-Range Correlations experiment conducted in 2021 at Jefferson Lab’s Experimental Hall D. This experiment was one of the first to use nuclear targets, rather than hydrogen. This makes the data particularly sensitive to axions produced through the Primakoff process, an electromagnetic process whose rate scales with the square of the nuclear charge, i.e., the production rate from scattering on a carbon nucleus would be 36 times higher than on a hydrogen nucleus.
The results of our axion search (in terms of coupling strength Λ) as a function of di-photon mass. The upper search compares our raw data to a background-only hypothesis. The lower search tests a background-subtracted spectrum.
In our analysis, led by MIT graduate student, Jackson Pybus, we searched our data for scattering events that produced a pair of photons, which may be the result of axion decay. The energies and angles of these photons can be used to reconstruct the mass of the hypothetical progenitor particle. We then searched the mass spectrum of signs of a resonance, a so-called “bump hunt.” Our results were statistically consistent with the background-only hypothesis, hence no discovery. Instead, the data allow us to exclude axions in a certain range of mass and coupling. Our limit is not yet world leading, but this is no surprise for an analysis of an experiment that was not designed around an axion search. We see several targeted improvements that could be made in a future dedicated experiment that would dramatically enhance our sensitivity.
Phase space for axion-like particles, in terms of mass and coupling strength (Λ). the gray regions have been excluded by other experiments. While our exclusion limits are not world leading, there are several improvements that could be made to a future experiment that would significantly enhance the sensitivity.
The most significant challenge with our current experiment is material in the path of the beamline downstream from the target. The production of the unstable eta meson (which decays into a photon pair) from collisions with downstream material had the potential to fake an axion like signal. This forced us to perform a background subtraction procedure that hampered our statistical sensitivity. Removing one of the detectors, the Forward Drift Chambers, would be a good start. But adding a downstream bag of helium, which displaces air and reduces the density of downstream material would be a game changer. With these improvements, a search for axions through Primakoff production would become world leading.
On June 3rd, Erin defended her PhD thesis, titled, “Probing the Isospin Composition of Short-Range Correlated Pairs at Jefferson Lab Hall B.” Erin’s graduate work encompassed a number of projects, all related to the phenomenon of “np-dominance,” the phenomenon that short-range correlated pairs are much more likely to form between a proton and neutron, rather than two protons or two neutrons. In addition to analyzing data from the CLAS e2a experiment, performing theory calculations using Generalized Contact Formalism, Erin was one of the leaders of the team that conducted the CLAS12 short-range correlations experiment in 2021–22, and helped to calibrate and analyze the data. Erin’s particular focus was on neutron detection. She applied her skills in machine learning to study the problem of discriminating signals produced by real neutrons from those produced by spurious charged particles (an example from Erin’s thesis is shown in the event display below). Erin was able train her machine learning models using samples of data from exclusive reactions, in which the neutron’s momentum vector could be inferred from other detected particles (see figure below, right).
Erin received her B.S. in physics from Le Moyne College in 2013, and her M.S. in physics from the University of Maryland. She joined the GWU Jefferson Lab Group in 2020 as a Columbian Distinguished Fellow. This year, she was named the Physics Department’s winner of the 2024 Berman Prize for excellence in experimental physics. Beyond her scientific accomplishments, Erin contributed in so many positive ways to this group, through her leadership, her mentorship, her fostering of a supportive team environment, and especially her tenacity at every physics problem she faced. As she moves to the next stage of her career she will be missed by all of us.
Our group’s latest paper, published in Physical Review C, examines inclusive electron scattering (only the scattered electron is detected) from helium-3 (two protons and one neutron) and tritium (two neutrons and one proton) nuclei. Comparing the scattering cross sections from these two nuclei in large Bjorken-x regime has been suggested as a method for learning about the relative rates of proton-proton, proton-neutron, and neutron-neutron short-range correlations. In our paper, we consider the problem using a theoretical spectral function (a probability distribution for finding a nucleon in the nucleus with a given momentum and separation energy) and find some problems with that approach. One problem, shown in the lower panel of the figure above, is that even at large Bjorken-x (xB>1.5), there is still a large contribution to the cross section from low separation energy (Es) nucleons. This means that we aren’t learning purely about pairs of correlated nucleons, but also about single nucleons and/or triplets. Distinguishing between those scenarios will require looking in a larger nucleus, such as Helium-4.
Group members Phoebe, Olga, and Axel travelled to Sacramento, CA this April to present their research at the APS April Meeting. Phoebe presented on the Hall D Short-Range Correlations / Color Transparency Experiment, and how the use of photoproduction reactions to probe short-range correlations in nuclei can help us verify that what we’ve learned about correlations does not depend on the reaction mechanism. Olga gave an update on her work on the Dalitz Analysis of the eta’ meson (GlueX), showcasing the agreement between the background-subtracted data, and a parameterized model of the decay. Axel gave an update on the BAND analysis (CLAS), as well as presenting results from a recently accepted paper on the phenomenology of short-range correlations in helium-3 (two protons and one neutron) and tritium (two neutrons and one proton).
In addition to these research talks, Phoebe and Olga helped organize the Jefferson Lab Users Organization’s (JLUO’s) satelite workshop, both speaking about their great work leading the Jefferson Lab’s Graduate Student and Postdoc Association. Olga also delivered an overview of the recent highlights from Hall D.
A short-range correlation of hummingbirds seen outside the convention center
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.
Peak cherry blossoms on the Potomac
Stumpy’s final bloom
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.
In a recent paper in Physical Review C, our group, in collaboration with GW professor Ron Workman and with Prof. Alfred Svarc from the Rudjer Boškovic Institute in Zagreb, Croatia, describe the latest release version of SAID partial wave analysis global fits. SAID consists of a data base of over 10,000 scattering measurements in a variety of observables and channels, as well as a simultaneous multi-channel partial wave analysis global fit, which allows predictions of the complete set of observables. In this paper, we have used the updated fits to produce revised estimates of the helicity amplitudes of light baryonic resonances. An example of data and the resulting fit for one observable (differential cross section) for one channel (gamma p –> π0 p) for one beam energy (E=1625 MeV) are shown below. One motivation for this update was the publication of the E double spin asymmetry results in the “gamma p –> π0 p” reaction that made up Chan Kim’s PhD thesis.
An example figure from the paper showing the differential cross section for the gamma p –> π0 p channel at a photon beam energy of 1625 MeV. World data are shown in blue points. The new SAID fit results are shown with the red solid curve, while other less recent global fits (SAID 2012 in blue, MAID 2007 in green, Bonn-Gatchina 2019 in magenta) are shown for comparison.
