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Richard Trotta, 05/20/2024 01:16 PM

Overview

Professor Zheng’s group conducts research at the Thomas Jefferson National Accelerator Facility (www.jlab.org, or see a short and cool video here). Our research interest includes study of the nucleon structure, with a focus on the spin structure of the nucleon measured using polarized beams and polarized targets. The nucleon structure is determined by how quarks and gluons interact with each other, thus such information could reveal some fundamental properties of the strong interaction and QCD. Experiments that studied this topic include previous 6 GeV A1n and EG4 experiments, the 12 GeV A1n experiment that was completed in year 2020, and the recently completed CLAS12 A1p (RGC) experiment. Another major aspect of our research is testing the Standard Model of Particle Physics by measuring parity violation (PV) in electron deep inelastic scattering (DIS) (see this Nature article on the subject, or for a lighter reading see Dr. Marciano’s Quarks are not ambidextrous). By measuring the PVDIS asymmetry off a hydrogen and a deuterium target, one can access important coupling constants of the Standard Model and study many interesting hadronic effects such as charge symmetry violation and the asymmetry in the sea quark distribution inside the nucleon. Looking forward, future projects that focus on this direction include PVDIS using the upcoming mid-scale equipment project called SoLID and the future electron-ion collider (EIC), The SoLID, if combined with a future positron beam at JLab, also offers the possibility of measuring a new electron-quark effective coupling that was never measured before.

Research

About Electron Scattering

When we try to study an object, the first thing we do is to observe it under natural light. What happens here is that the light probe (electromagnetic waves in the visible frequency range) scatters off the surface of the object and enters our eye, which is then transmitted to the brain by optical nerves. To study the nucleon is not as simple. The mass of a nucleon (proton or neutron, see figure to the right) is about 1 GeV/c2, or 10-27 kg (or 10-35 of the empire state building). To study such tiny objects, ordinary tools (your naked eye, optical microscope) do not work. What people have been doing is to scatter high-energy beams of electrons or photons (our “probe” for the tiny world) off target made of protons or few-body nuclear systems. The scattered electrons and other particles ejected from the reaction are detected in various detectors (our “sensor”), and the detector signals are transmitted to computers (our “brain”) for further processing. The probabilities for certain scattering process to happen (called “scattering cross sections”) are extracted, and they contains information on the internal structure of the nucleon or nucleus.

Using high energy photon or electron beams to study the nucleon is not only a useful tool, it is also a very “clean” tool: When we use electron beams, what actually happen is that the electrons would exchange a virtual photon with the target (see figure), in other words, the electron interacts with the target through electromagnetic interactions. The electromagnetic interaction is the best understood among all 4 interactions, tested experimentally to 10-9 precision. (Imagine that you have a bank account of $10M and you know where every penny is spent, that is 10-9 precision). Because we know our probe so well, the detected signals would almost unambiguously reflect only the structure of the target.

About the Structure of the Nucleon

What can we do once we look into the nucleon, and what information do we learn? Among the four interactions of our universe, three are described by the Standard Model of Particle Physics. While Electromagnetic interaction is the best understood, the nuclear strong force (or strong interaction) is the least understood one. A natural laboratory in which we can study strong interaction is the nucleon: since nucleons are made of quarks interacting primarily through strong interaction, measuring how quarks form the nucleon can help us understand strong interaction. Such measurements typically include the nucleon structure functions, the parton distribution functions (PDFs), and generalized parton distributions (GPDs).

In many experiments, spin polarizations of the beam, target and/or the ejected particles are used to extract additional information, such as how quarks inside the nucleon carry the nucleon spin. These are call doubly polarized scattering experiments. The scattering cross section for such experiments depend on how the spin of the incident electron is aligned w.r.t. the target spin (imaging throwing two magnets together, how they react depends on how their N/S poles are aligned). Measurements of such polarized cross sections or “asymmetries” provide additional, rich, degree of freedom in our study of the strong interaction. Citing Bjorken1: “Polarization data have often been the graveyard of fashionable theories. If theorists had their way, they might well ban such measurements altogether out of self-protection.” Such polarization data are what we are after.

About Parity Violation Electron Scattering (PVES)

In addition to electromagnetic interactions, the electron beam can also weak interactions with the target nucleon or nuclei. In this case, a Z0 boson – one of the carriers of weak interaction or the weak nuclear force – is exchanged between the electron and the nucleon. Because parity symmetry is violated in weak interaction, the cross section is slightly different between right-handed (spin parallel to momentum) and left-handed (spin anti-parallel to momentum) electrons. The small asymmetry between the two cases is precisely predicted by the Standard Model of Particle Physics.

The weak interaction is not as well tested as the electromagnetic interaction, and one expects precision measurements of the small PVES asymmetry may reveal something new: If the measured value differs from the value predicted by the Standard Model, we say that there is indication of New Physics beyond the Standard Model. As for why we should expect the existence of such New Physics? Let’s just say that as physicists (and with good knowledge and understanding of relativity), many of us would believe the only thing absolutely true is ignorance of the human race: There is always unknown, or something new, beyond what we already know. Do you think quarks and electrons are the smallest particles of the universe? Think again! That chart of Standard Model surely looks a lot like Mendeleev’s periodic table, and look what Mendeleev’s table told us about the structure of atoms.

About Thomas Jefferson National Accelerator Facility

For both nucleon structure study and PVES study, high precision is the key. To achieve high precision, we typically need a LOT OF electrons scattering off a VERY THICK target, just to increase the event counts within a reasonable time. The continuous-wave (CW) electron beam accelerator facility (CEBAF) at the Thomas Jefferson National Accelerator Facility (also called JLab) turns out to be the best facility in the world for the high precision measurements that we want to do. Thus our research program is primarily carried out at JLab, located in Newport News, VA, only 2-1/2 hours drive from Charlottesville.

About the Electron-Ion Collider (EIC)

The electron-ion collider (https://www.bnl.gov/eic/), currently planned to be built at Brookhaven National Laboratory, is the future facility aiming at further understanding the strong interaction and QCD. The center-of-mass energy and the luminosity of EIC reside between those of JLab (higher luminosity, lower energy) and of high-energy facilities such as HERA (now retired) and the ongoing LHC at CERN (lower luminosity, higher energy). The EIC will thus open up a new era in both kinematic coverage and precision for nuclear physics research in the US and worldwide.

Research Spin-Offs

Spin-polarized nuclear fusion

Nuclear fusion has long been regarded as the “holy grail” of energy production that would provide a clean, renewable, and powerful source of energy for the future. Despite decades of research, however, we still have not reached “ignition”, defined as self-sustained energy production, in any of fusion reactors built so far. Under the current circumstances, all possible mechanisms that could boost the fusion reaction rate should be studied and pursued. One such mechanism is the use of spin-polarized fuel. It was predicted2 that the fusion cross section between deuterium (D) and tritium (T), or 3 He and D, is boosted 50% when fully polarized fuel is used. This initial boost could raise the plasma temperature, which increase further the fusion reaction rate, and thus could have a determining effect on whether a reactor reaches ignition. However, the feasibility and the effectiveness of polarized fusion have never been proven experimentally.

One “spin-off” of our research is to demonstrate the ability to fill inertial confinement fusion (ICF) polymer pellets with pressurized polarized 3He with high purity, to maximize the in-pellet polarization, and to maintain the polarized state long enough to allow the pellets to be injected into a fusion reactor and for fusion to occur. Our ultimate goal is to conduct the first proof-of-principle, in-situ test of polarization survival and polarization dependence of D- 3 He fusion using the DIII-D Tokamak in San Diego.

Projects

Ongoing/Upcoming Ph.D. Projects

The following is a list of experiment we are committed to. On the other hand, experimental run schedule is fluid, and we strive to assign each PhD candidate a high-quality project that is aligned with the group’s research focus, while also ensure a reasonable graduation time. Therefore, new experiments and/or projects can be arranged and discussed when new members join the group.

  • We anticipate that in 2024 or in the near future, we need two full-time graduate students to carry forward spin-polarized fusion work, in collaboration with other JLab and UVA colleagues:
    • one will be stationed at JLab and develop “frozen” polarized HD pellet
    • one will be working with Prof. Miller from SOM to develop polarized 3He pellet

From the News page you probably have read that some of our graduate students were awarded the DOE SCGSR awards. Other possible ideas for SCGSR application (co-mentored by JLab staff) include:

  • high-rate, radiation-hard detector and DAQ system development for the SoLID project
  • projects towards building up a positron beam at JLab

Each of above can be combined with experimental data analysis or other work of JLab experiments, including SoLID, to form a full PhD thesis.

Projects for Undergraduate Students

The group does not have a “standard list” for undergraduate students, but we will work with students interested in research on an individual basis. Choice of topics often depends on the interest and strength of the student, as well as what we have on-going within the group.

As an example, we had the following projects for summer 2021 through summer 2023 and their current status. Publications for which our students are listed as co-authors are shown in bold. If a followup project is needed or ongoing, they are shown as bold italic. 

SoLID detector beam test data analysis, PID performance study, ML/AI application in PID — Darren Upton, summer 2023 SULI project focusing on AI/ML PID; and Spencer Opatrny focusing on classicial PID; Next step is to apply ML/AI PID on data samples
    Derivation of general DIS formalism that includes arbitrary polarization of both the lepton and the nucleon — completed by Paul Anderson (DOE SULI summer 2022), and numerical calculation by John Grant (Mitchell fellowship summer 2023). Next step is to publish results on the transverse-electron spin parity violation asymmetries, paper writing in progress;
    FOM study of an A1n measurement using a 22 GeV Beam and Hall C’s SHMS+HMS at JLab — completed by Cameron Cotton in summer 2021 and the extension to A1p and quark polarization was completed by Cameron Cotton and Jesse Smith in summer 2023; DIS Proceedings submitted;
    Projection of A1p and A1n measurements at the EIC and extraction of the strong coupling constant using the Bjorken integral — near completion by Darren Upton, paper under collaboration review;
    Extraction of neutron g1 from CLAS6 EG4 experiment — nearly completed by Darren Upton and Cole Faggert, paper under collaboration review;
    Impact study of PVDIS using SoLID and a 22 GeV Beam at JLab, and PVDIS at EIC — completed by Alex Emmert (deuteron, UVA 2022 Summer Mitchell Fellowship), Kiernan Riesing (deuteron, DOE SULI Sp2022), and George Evans (proton, DOE SULI Summer 2022), results included in the SoLID white paper, published in J. Phys. G in 2023. 
    PDF uncertainty study and projection for EIC PVDIS — completed (as part of the EIC ECCE Detector Proposal) by Alex Emmert and Michael Nycz (summer/fall 2021), published in Phys. Rev. D. 
    Feasibility/FOM study of a measurement of Ae+e- at EIC — completed (as part of the EIC ECCE Detector Proposal)
    Beam helicity structure at EIC and implication on data analysis — dis-continued (too simple to be a project)

For Fall 2023 and forward, we have the following bucket list of very rough ideas and target milestones. 

Hardware design and JLab positron source related:
        track spin rotation (precession) of electron and positron beam through CEBAF and LERF; keywords: spin rotation, spin precession, positron source; We anticipate this work needing 1-2 summer undergraduate students, and a part-time graduate student.
    SoLID-related:
        ML/AI application in PID and tracking for SoLID; keywords: PID, AI/ML;
        pulse-shape simulation; keyword: simulation;
        one possible project is to code the user routine of generator Djangoh to interface with SoLID’s GEMC (GEANT-based) simulation code; keywords: Djangoh, LUND format, GEANT;
        3D-printing a miniature SoLID
    Simulation study: PVDIS radiative corrections, comparison of Mo&Tsai and Djangoh’s internal bremstrahlung radiation function; keywords: radiative corrections, Djangoh
    (Projection: SoLID projection for inclusive spin physics, possibly reserved for summer 2024, can serve as the stepping stone to a rungroup proposal)

1 QCD: Hard collisions are easy and soft collisions are hard, J.D. Bjorken, NATO Sci. Ser. B 197, 1 (1987).

2 Fusion Reactor Plasmas with Polarized Nuclei, R. M. Kulsrud, H. P. Furth, E. J. Valeo, and M. Goldhaber, Phys. Rev. Lett. 49, 1248 (1982).

Updated by Richard Trotta 6 months ago · 13 revisions