Graduate students win a trio of fellowships

23/03/202323/03/2023

Graduate students win a trio of fellowships

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.”

29/09/202229/09/2022

Town Hall on QCD: why we should accelerate positrons

This past weekend in Cambridge, MA, was one of several town hall meetings being held around the country to get input from the community as NSAC (Nuclear Science Advisory Committee) undertakes the next “Long-Range Plan.” These plans are developed every seven years or so and set the priorities for nuclear physics research funding by government agencies such as the U.S. Department of Energy and the National Science Foundation. This weekend’s town hall was on Quantum Chromodynamics (QCD), and I had the privilege to be invited to talk about physics that would be made possible with an accelerator that could accelerate positrons.

Accelerating positrons isn’t hard. An existing electron accelerator, such as Jefferson Lab’s CEBAF, can do the job, provided the polarities of all the steering magnets are reversed. But it’s making the positrons in the first place that requires some investment. It’s not cheap (in the ball park of tens of millions of dollars), but it’s not nearly as expensive as, say, upgrading CEBAF’s terminal energy from 12 GeV to 24 GeV. And it’s no where near as expensive as building a new accelerator, such as the upcoming electron ion collider. As part of the Jefferson Lab Positron Working Group, I am advocating that a positron source be built at Jefferson Lab, so that positrons could be accelerated using th CEBAF accelerator, and experiments could be conducted with all of the amazing detectors and equipment that are already part of the laboratory’s infrastructure.

There are many great experiments one can do with a positron beam, and among them, the application I find to be most critical is determining how much two-photon exchange occurs in electron scattering. Electron scattering is a great probe for learning about how quarks are distributed within protons, but in analyzing such measurements, we assume that the electron exchanges one and only one virtual photon with the quark that it collides with. It can exchange more than one photon (with each additional photon being significantly less likely), but calculating the probability of a second exchanged photon is not straightforward, and we typically assume that it has a negligible impact on the results. This may not have gotten us into trouble everywhere, but it likely has gotten us into trouble in our efforts to measure the proton’s charge distribution (its so-called “electromagnetic form factors.”)

Positrons can help clear up this mess because two-photon exchange has the opposite effect on positron scattering as it does on electron scattering. If we can measure a difference between, say, a positron-proton scattering reaction, and an electron-proton scattering reaction, then it tells us exactly what the effects of two-photon exchange are. This could help clear up the proton’s form factors, and tell us what to watch out for as we embark on measuring challenging new reactions like deeply virtual compton scattering or deeply virtual meson production.

Positrons are great for answering lots of other questions too, such as helping to map out the protons 3D structure (via its Generalized Parton Distributions), probing for “strange quarks” in the proton, or even searching for light dark matter. I think the benefits are really clear, and cost is not exorbitant, and I hope that a positron source at Jefferson Lab is endorsed in the Long-Range Plan.

If you’d like to learn more, I highly recommend checking out the JLab Positron Working Group’s White Paper, published as a special topical issue of the European Physical Journal A.

14/02/202217/02/2022

Undergrad advisee featured in GW Today news article

GW Today featured one of my undergrad advisees, Gabe Grauvogel, in a news article, and talks a little bit about the kind of exciting research in nuclear physics that undergraduates can contribute to. This is a great credit to Gabe’s hard work and enthusiasm for physics.

16/03/2021

“Extracting the number of short-range correlated nucleon pairs from inclusive electron scattering data” published in Physical Review C

Graph showing kinematic envelopes of quasi-elastic scattering from deuterium and carbon — The kinematic envelope relating Bjorken-x and minimum nucleon momentum is subtly different for deuterium and carbon, depending on both nucleon motion and residual excitation energy.

Comparisons of inclusive quasi-elastic cross sections in “x_B>1 kinematics” have long been interpreted as a measure of the relative number of high-momentum nucleons in different nuclei. This interpretation relies on a the effect of a kinematic envelope relating the inclusive scattering parameter Bjorken-x, x_B, and the minimum nucleon momentum. In our latest paper, “Extracting the number of short-range correlated nucleon pairs from inclusive electron scattering data,” we argue that the picture is a bit more subtle.

In quasi-elastic scattering (in which the electron ejects an intact proton or neutron from the nucleus) not all combinations of x_B and nucleon momentum legally conserve energy and momentum. In order to satisfy this conservation, scattering at high x_B can only happen from a nucleon with high momentum. There is an envelope (shown in the figure above), above which the reaction can proceed, below which it is forbidden.

To determine the number of high-momentum nucleons in a nucleus, say carbon, one could compare the high-x_B inclusive cross section in electron-carbon scattering to the same quantity in electron-deuterium scattering. By requiring high-x_B, one is directly probing high-momentum nucleons. The ratio of cross sections can be interpreted as a ratio of the number of high-momentum nucleons in the nucleus.

However, we point out that nuclear effects, such as the way the high-momentum of the nucleon is balanced, as well as the residual excitation energy of the nuclear remnant after the scattering interaction can change the kinematic envelope. We argue that such effects can distort any extraction.

We follow this up with calculations, built around the tool called “Generalized Contact Formalism” that show the wide-range of combinations of nuclear parameters that are consistent with the data from previous scattering experiments.

This paper was a result of the hard work of several collaborators: Hebrew University (Jerusalem) graduate student Ronan Weiss, as well as MIT graduate students Andrew Denniston (pictured below) and Jackson Pybus.

10/03/202110/03/2021

“Probing the core of the strong nuclear interaction” is featured as a DOE Science Highlight

Individual electron-scattering collisions are like individual still frames capturing the rapidly changing motion of protons and neutrons in the nuclus.

The U.S. Dept. of Energy’s Office of Science has featured our recent paper, “Probing the core of the strong nuclear interaction” (Nature 578, pp. 540–544, 2020) as one of its Nuclear Physics Science Highlights. The highlight, titled “Researchers Overcome the Space between Protons and Neutrons to Study the Heart of Matter” recognizes this work as one of the most significant recent achievements from the nuclear physics research sponsored by the Dept. of Energy (DOE). In addition to supporting the research, the experimental data in this paper were collected at the Thomas Jefferson National Accelerator Facility (JLab), a DOE-operated facility.

31/10/2020

APS Division of Nuclear Physics (Remote) Fall Meeting

The group was well-represented at this year’s APS Division of Nuclear Physics Fall Meeting, with Phoebe, Sara, Tyler, Axel, and group alum Holly all giving talks about their latest research results. But for CoVID, the meeting would have been held in New Orleans, but it seamlessly transitioned to Zoom.

In her first APS talk, Sara presented her work on the upcoming LAD experiment, in which a measurement of deep inelastic electron-scattering on neutrons deuterium—with the crucial addition of a “tag” on the spectator proton—can help test the connection between the EMC Effect and short-range correlated nucleons. Sara presented her studies of simulated backgrounds, which will help optimize electron spectrometer settings. In the same session, Tyler also presented the latest analysis of the complementary BAND experiment.

In her first APS talk, Phoebe presented the upcoming SRC@GlueX Experiment in Jefferson Lab Hall D, in which short-range correlations will be probed using a high-energy photon beam. She showed her latest simulation studies helping to plan and optimize the run, which will begin in August, 2021.

We’re all looking forward to in-person meetings in the future, but until then, stay safe, wear a mask, keep social distance!

20/09/2020

Tyler delivers GW’s Physics Dept. Colloquium

This past week, the Physics Department Colloquium Series resumed for the fall semester, though with Covid restrictions it will be an online affair. Tyler was the opening speaker, and talked about the connections between his PhD work—investigating the structure of the neutron by scattering electrons from helium-3 and tritium—and his current work—investigating how proton and neutron structure can change within nuclei. The two topics are inexorably linked. Since there is no way to make a free neutron target, one can only study neutrons within nuclei. But at the same time, one needs to understand the structure of the neutron to study nuclei. Tyler’s work on the BAND experiment and (upcoming) LAD experiment will hopefully give us a clear picture of the neutron’s (and proton’s) structure inside nuclei.

21/04/202021/04/2020

New Generalized Contact Formalism result published Physics Letters B

Generalized contact formalism matches data for the relative abundances of proton-neutron and proton-proton correlations in Helium-4.

Our group has a new result using the Generalized Contact Formalism, which has proved so successful in describing short-range correlations between nucleons. MIT graduate student Jackson Pybus, performed calculations corresponding to a measurement made in 2011 in Hall A of Jefferson Lab of the triple-coincidence (e,e’pN) reaction on a helium-4 target. The GCF calculations match the experimental measurements without any free parameters, and further confirm the acceptance corrections made in the original analysis. This is one more example of GCF proving effective at describing the electron-induced knock-out of correlated nucleons, following previous results in PRL and in Nature on carbon.

MIT graduate student Jackson Pybus, lead author of this work

I’ve been fortunate to work with Jackson on a number of projects starting when I was a postdoc at MIT, and I’m thrilled that we’ve continued to collaborate since I’ve come to GW.

21/04/202021/04/2020

Phenomenological approach to two-photon exchange published in Journal of Physics G

In a new paper in Journal of Physics G, I argue that three recent experiments measuring the positron-proton to electron-proton elastic cross section ratio can make no definitive statements about proton form factor discrepancy because even the size of the discrepancy is not well-constrained. It is widely believed that the discrepancy between polarized and unpolarized measurements of the proton’s form factors is caused by the effects of two-photon exchange (TPE). Three recent experiments—one at the VEPP-3 Storage Ring in Novosibirsk, one using the CLAS detector at Jefferson Lab, and finally the OLYMPUS Experiment at DESY in Hamburg—looked for two-photon exchange by looking for any difference between the positron-proton and electron-proton cross sections. The measurements found only modest differences, and the results have been variously interpreted as supporting or contradicting the TPE hypothesis. Using a phenomenological approach, I estimate the e+p/e–p ratio one would need to resolve the discrepancy, as well as, for the first time, an estimate of the uncertainty. I find wide variation depending on what one assumes about unpolarized form factor measurements. The TPE hypothesis is neither confirmed nor denied, and future measurements at higher momentum transfer are needed.

The OLYMPUS experiment saw only slight differences between the e+p and e-p cross sections, but this is perfectly consistent with the small discrepancy at the accessible momentum transfer.

06/03/2020

New paper in PRL on the quark structure of neutrons

Tyler and I, along with our collaborators at MIT, Old Dominion, Penn State, Tel Aviv, and Jefferson Lab have just published an article in Physical Review Letters on the quark structure of free neutrons. Whereas free protons are easy to study (using my favorite technique, electron scattering!), neutrons decay with a lifetime of about 15 minutes, and can’t be practically made into a target. Instead, we learn about the neutron by scattering from nuclei, like deuterium or helium, and try to account for any effects that come from nuclear binding. One problematic effect is that the nuclear environment seems to change the structure of protons and neutrons, which we’ve given the name “The EMC Effect.”

In this work, first-authored by MIT graduate student Efrain Segarra, we looked at electron-scattering data from lots of different nuclei, including heavy nuclei, where the EMC Effect is large. Then, using the model developed in one of our recent papers, in which the EMC Effect stems only from short-range correlated proton-neutron pairs, we inferred what the free neutron structure must be to explain the data. This has consequences for the results of the MARATHON Experiment, which will hopefully be published soon.

If short-range correlations are responsible for the EMC Effect, then heavy nuclei would indicate a larger F2n/F2p ratio at large x.

Our paper is a quick read, and I hope you enjoy it.