• Table of contents
• Overview
• Research
  • About Electron Scattering
  • About the Structure of the Nucleon
  • About Parity Violation Electron Scattering (PVES)

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.