What’s the Matter with Matter?


Jefferson Lab (left triangle) in Newport News Virginia.

Next week I will be attending the Hampton University Graduate Summer Program (HUGS), which is a three week long series of lectures and activities at the Thomas Jefferson National Accelerator Facility (Jefferson Lab or JLab) run by the US Department of Energy (DOE). The summer school will “focus on experimental and theoretical topics of high current interest in strong interaction physics. In particular, students will receive excellent insight into the physics to be studied at Jefferson Lab and at the Electron-Ion Collider (EIC) in the upcoming years, as well as related topics of interest in nuclear and particle physics.” The main topics will be about nuclear physics in theory and in experiments at particle accelerators like JLab and the upcoming EIC.

Schematic drawing and explanation of the continuous electron beam accelerator facility (CEBAF) particle accelerator at JLab.

My Ph.D. research and experimental work will primarily focus on future JLab experiments. I will be designing, running, and analyzing experiments at JLab that use polarized electrons to isolate out the weak force part of electron-electron or electron-neutron interactions, which are the parts of those interactions that use an intermediate Z boson rather than a photon as the force mediating particle. JLab is ideally suited to achieve high precision measurements to test the predictions of the Standard Model of Particle Physics, even to the degree that we have the ability to test for the existence of new forces and physics in these experiments beyond just the photon and Z, and we can even test some parameters relevant for neutron star models.

Parity violation asymmetry : Tree level Moller asymmetry : Qw.
Schematic overview of some of the math that goes into predicting what the weak mixing angle coupling strength should be, ignoring higher order corrections.
Plot of the projected JLab MOLLER experiment measurement of the weak mixing angle coupling strength alongside older, less precise measurements at different energy scales.
The idea behind vacuum polarization.

The Weinberg mixing angle (actually the square of the sine of the angle, for reasons having to do with spontaneous symmetry breaking and the Higgs mechanism in the Standard Model) is shown plotted on the right with some of the old measurements as well as the expected value from the upcoming JLab experiment called MOLLER (standing for Measurement Of a Lepton Lepton Electroweak Reaction, also conveniently named since the electron scattering process under investigation is Møller scattering). As you can see, this weak force coupling strength is a function of energy, where the μ horizontal axis has units of GeV, which is the unit of energy used in particle physics where 1 GeV corresponds approximately to the mass energy of a proton or neutron (recall that E² = (mc²)² + (pc)², which is the famous E = mc² when the momentum p = 0). Not only is the coupling a function of energy, but it is kind of a weird one too. The dip and kink at 91.2 GeV corresponds to the production of real Z bosons in the interaction, since that is their mass and the physics is heavily dependent on the mass of the force mediator. On the left of that dip is an effect called charge screening, which is similar to the kind of screening we are familiar with from classical electric charges getting screened when they are in some medium. In quantum field theory it is possible for a pair of plus and minus charged particles to pop out of the empty vacuum for a very brief amount of time, and these vacuum excitations will then act as a polarizing medium to screen a bare charge. The higher the momentum of a particle coming in to interact with a screened charge, the closer in that particle can get before being repelled away again, then the closer in that particle got, the less screened the charge was. So in general we say that as the energy of a process increases you are able to change the strength of the couplings of the forces involved. On the right side of the kink in the plot is another effect called anti-screening, which is due to the fact that now that there is enough energy to actually create real Z bosons themselves, now the screening effect from before has Z’s involved too, except that now the Z’s carry the weak charge differently than before and so it behaves differently.

Because the weak force mediator that we are sensitive to, the Z boson, is a 91 GeV mass particle, it means that if we see more or less signal than we expect in our measurement at relatively low momentum transfers (energy scale μ ~ 1 or 0.1 GeV) that we actually are sensitive to the kinds of physics going on at the μ ~ 100 GeV scale, which is the kind of stuff the LHC is trying to see now (for instance producing real Z’s at 91 GeV or the recent discovery of the Higgs at 125 GeV). We are sensitive to new weak scale physics, which is prime territory for stuff that hasn’t been ruled out yet by the LHC’s direct production approach and is where theorists prefer to put new forces and new physics (so that it would be new compared to the Standard Model but still a low enough energy scale that we could still hope to be able to see it without spending too much money, while still matching desired predictions for stuff like new dark matter or supersymmetry forces). Ideally, if we were to see some significant deviation from the Standard Model’s expectations then we would say it could very well be due to a new 5th force, though we wouldn’t be able to really say much more about it without having to do further tests.

The Standard Model of Particle Physics with all of the particles as we currently understand them.

If we were to make a discovery of such a discrepancy, then it kind of has to be Beyond the Standard Model physics (BSM) for a number of reasons. Dark matter results from cosmology are to be taken seriously, and we definitely know that the Standard Model completely missed the origin of neutrino masses, and it doesn’t explain why any of the coupling strengths or masses are the way they are or why they are often so vastly different in scale from each other. Of course everything could just conveniently work nicely to give us a universe like what we see, but there are some ways of making simpler, higher theories that actually give rise to the Standard Model we see. Unfortunately for theorists, or fortunately for the Standard Model, a lot of their immediate predictions continue to not be seen as the predictions continue to line up nicely with the Standard Model with only a few additions to account for possible sources of neutrino masses. This lack of new signals heavily constricts the available theories down to a harder and harder to measure set of options.

Schematic portrayal of the potential unification of the forces.

The kind of work I will be doing at Jefferson Lab for my Ph.D. will therefore be an attempt at overthrowing the Standard Model, but really only in the sense that we are trying to look deeper than, beyond the Standard Model. Just like how classical mechanics and Newtonian gravity is correct, but relativity is the underlying reality that gets approximated to classical mechanics in low energy and slow velocity cases. Similarly the Standard Model is definitely true in the low energy cases, but actually changes and needs more information to be predictive at much much higher energies. Of course we could eventually find enough information to totally revolutionize our understanding away from the Standard Model, but for now the idea is to understand how it arises as a limiting case of an even more fundamental theoretical framework, generally referred to as a Grand Unified Theory (GUT) or Quantum Gravity.

By using low energies with high quality polarization we are able to see signals that would otherwise be too small from the suppression due to difference of energy scales. Weak scale interactions (around the 91 GeV scale of the Z boson mass) actually do play a role at all energy scales, but they fall off in importance at around 1/(E – mass of the Z)², which is a significant, basically 1/Mz² reduction at low energies away from the scale of the weak energy scale where we expect to still be able to find new physics. We are using the opposite polarization subtraction trick to get rid of the large elctromagnetic signals and then just stare at the small signal precisely enough to make new discoveries, and since we can test Large Hadron Collider scale kinds of physics much faster and with cheaper equipment, that is part of what makes this work so exciting.

If you would like to know more about my research, feel free to ask me anything. There are tons of details that I completely skipped over, particle physics and quantum field theory are large and complicated subjects, but they are also very interesting and can be intuitive if taken slowly. You can also look at an old presentation I gave going over the things I just discussed in more detail, though it is probably less clear.


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