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.
In a new paper published in Physical Review C, Axel Schmidt and collaborators from MIT explored the onset of the short-range correlated regime for protons inside nuclei. At any given moment, most protons inside nuclei feel strong attraction from many nearby nucleons (other protons and neutrons). However, some protons happen to be very close to a nearby neighbor nucleon, enter a short-range correlated configuration, in which the forces between the paired nucleons are much much stronger than the forces from the rest of the nucleus. These correlations produce states of very high relative momentum, which is typically how we identify them in electron scattering experiments. One such approach is to examine the momentum of electrons after they have scattered from a nucleus. If an electron leaves the collision with significantly more momentum than would be expected, given their scattering angle (high Bjorken-x, xB), then it is likely the electron hit a high-momentum nucleon participating in a short range correlation.
In this paper, we consider a slightly different technique: knocking out a proton from the nucleus. The additional information from the proton momentum vector allowed us to reject collisions with additional undetected particles, which are a background hiding our short-range correlations signal. We showed that this technique works by verifying the scaling in Bjorken-x seen in the carbon-to-deuterium cross section ratio; only we see it over the full range of x, shown in the figure above.
Detecting the knocked out proton has an additional advantage: an extra handle on what the proton’s momentum was prior to the collision, a variable we call missing momentum or pmiss. By selecting clean proton knock-out using missing mass, we could dial in different proton momentum regions using missing momentum. This allowed us to get an estimate of the width of the transition region between the uncorrelated and correlated regimes, shown in the figure above.
Future experiments, such as those in inverse kinematics, or those detecting low-energy recoiling nucleons (such as the recently approved ALERT experiment at Jefferson Lab) will be able to enhance the precision of our understanding of this transition region, and on how short-range correlations form inside the nucleus.
We’ve reached the end of a busy semester for our group, and as we transition into summer, it feels like a good time to recognize some of the achievements of our group members. First, our group was well represented among this year’s departmental awards.
Erin was awarded the Chair’s Prize for best physics poster by a graduate student at the GW Research Showcase.
Gabe was awarded the Berman Prize for “Excellence in Experimental Physics.”
Logan was awarded the Gus Weiss Prize for an “Outstanding Student in Physics.”
Gabe was also named a Columbian College Distinguished Scholar at this year’s graduation.
The workshop was held in the Curie Auditorium of the Centre National de la Recherche Scientifique (National Center for Scientific Research), quite an impressive venue.
This semester also saw the graduation of two of our undergraduate students, Gabe and Logan. This felt monumental because they were Axel’s first undergraduate mentees at GW. Both Gabe and Logan wrote very impressive undergraduate theses, presented super research posters at the showcase, and were awarded departmental honors at graduation by unanimous acclamation of the faculty.
LoganGabe
Both Gabe and Logan will be missed, but they have exciting careers in physics ahead of them. Logan will be starting in the PhD program at Notre Dame this fall, and Gabe will be pursuing a PhD at Michigan State. We look forward to seeing all they discover.
Gabe is also a very talented percussionist. Here he is, playing four-mallet marimba with the GW percussion ensemble. So he has options, just in case physics doesn’t work out. 😉
This year, three graduate students in our group have won outside fellowships supporting their research into short-range correlations and hadron-structure modification in the nuclear medium. That three different agencies all elected to fund our students’ proposals speaks both to the talent and productivity of our students as well as the importance of their work.
Erin, Sara, and Phoebe next to the GW Hippopotamus
Erin Seroka wins a 2022-23 Jefferson Lab JSA Graduate Fellowship
Erin was named one of the winners of a 2022-23 Jefferson Lab JSA Graduate Fellowship, supporting her work investigating the isospin structure of short range correlations. Erin hopes to show that the observed rise in prevalence of proton-proton short-range correlations with missing momentum is accompanied by a decrease in the prevalence of proton-neutron short range correlations. Her analysis of data from the CLAS12 Short-Range Correlations Experiment has required a huge investment of time and effort into understanding the performance of the CLAS12 Central Neutron Detector, and has made her one of the collaboration experts on that detector.
Sara Ratliff wins a 2022-23 Center for Nuclear Femtography Graduate Fellowship
Sara has won a fellowship from the Center for Nuclear Femtography supporting her work researching the motion of quarks inside bound protons and neutrons. Sara’s research uses the novel technique of “spectator recoil tagging,” using the simultaneous detection of a neutron that was merely a spectator to a nearby violent deep inelastic scattering collision to learn about the initial state of the struck nucleus or nucleon. Sara uses the CLAS12 Backward Angle Neutron Detector (BAND) to detect neutrons and has become a critical member of BAND team, working understand the efficiency and performance of the detector.
Phoebe Sharp wins a 2022-23 US Dept. of Energy, Office of Science Graduate Fellowship
Phoebe was named one of the winners of the 2022-23 US. Department of Energy Office of Science Graduate Research Fellowships, supporting her proposal to learn about short range correlations using the novel technique of rho meson photo-production. Instead of using the conventional method of quasi-elastic electron scattering to break up a short-range correlated nucleon-nucleon pair, Phoebe’s thesis experiment used a high energy photon beam. Phoebe is investigating signatures of pair break-up through the detection of a highly unstable rho-0 meson. Short-range correlations have never been observed in photon-induced reactions, and Phoebe hopes not only to break new ground in detection, but also confirm that previously seen properties of short-range correlations are in fact “reaction independent.”
Sara, presenting her work on the EMC Effect and the BAND ExperimentPhoebe, talking about the Hall D SRC/CT Experiment, which used the GlueX spectrometerGabe, presenting in the undergraduate research session as part of the CEU programLogan presented a poster on his Hall D analysis as part of the CEU program
Phoebe in the Hall C counting house with collaborator Carlos Ayerbe Gayoso (William and Mary)
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.
The “missing energy” spectrum measured for electrons knocking out protons from Helium-3 (red) and Tritium (i.e., Hydrogen-3, black) as measured in a 2018 experiment. The helium spectrum shows a peak at 5.5 MeV, corresponding to collisions leaving behind a bound deuteron. A similar peak in tritium would imply a bound di-neutron state.
In a paper recently published in Physics Letters B, we analyzed data from a 2018 experiment to search for the possibility of two neutrons forming a short-lived bound state. The experiment was conducted to study how protons move in helium-3 (two protons and one neutron) and tritium (two neutrons and one proton) nuclei. We realized that the data could reveal the presence of a bound di-neutron. Our analysis showed no evidence of any such state.
When a proton is knocked out of helium-3, one proton and one neutron are left behind. These can fly apart, or they can stay bound as a deuterium nucleus, called a deuteron. By contrast, when a proton is knocked out of tritium, two neutrons are left behind. No one has ever observed a “di-neutron” state, and so it is believed that the two neutrons always fly apart. However, if in some small fraction of collisions they could bind, then the signature in the data would look exactly like the deuteron signature in helium-3. The helium-3 data provide a sort of “template” that allowed us to search of a di-neutron without having to model exactly how it would appear in our detectors.
The exclusion limits placed by our search. The search becomes less sensitive for smaller hypothetical dineutron binding energies.
Looking at the data, we saw no evidence for a bound di-neutron state. This allowed us to place exclusion limits on possible di-neutron formation in tritium.
A schematic showing the bending and focusing magnets of the MUSE beamline
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.
The total measured photo-production cross section (gold points) on the proton is compared to the cross section for single pion production predicted by the SAID (red), MAID (blue), and Regge (cyan) models. The single pion contribution is dominant at lower photon energies.
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.
The convergence of the GDH sum rule (an integral over incoming photon energy) as a function of photon energy. The single-pion contribution nearly converges the sum rule for protons (left), whereas it only gets about 60% of the sum rule for neutrons (right).
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.
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.
Fig. 4a from the paper, produced by Gabe, showing the projected statistical uncertainties of the future experiment in comparison to a theoretical model developed by GW’s own Prof. Afanasev.
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.
