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.
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.
