From Muons to Antimatter: Two UMass Physics Grad Students Among Winners of Prestigious Department of Energy Award

The U.S. Department of Energy (DOE) recently announced this year’s batch of Science Graduate Student Research (SCGSR) Program awards. Among the awardees are two UMass Amherst physicists-in-training: David Kessler and Tristan Winick, both of whom are having their dissertations overseen by David Kawall.

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The SCGSR is a prestigious award, given to a host of grad students each year, which provides its awardees with supplemental funds to conduct part of their thesis research at a host DOE laboratory in collaboration with a DOE laboratory scientist. The DOE chooses students whose research projects are of “significant importance” and that “address societal challenges at the national and international scale.”

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“The DOE Office of Science provides the scientific foundation for solutions to some of our nation’s most complex challenges, and now more than ever we need to invest in a diverse, talented pipeline of scientists, engineers, and entrepreneurs who can help us build a brighter future,” said Harriet Kung, deputy director for science programs in the Office of Science. “These outstanding students will help us tackle mission-critical research at our labs as this experience helps them begin a successful and rewarding career.”

How to make one of the most precise measurements of all time in particle physics (hint: you need a really good ruler)

David Kessler, a fifth-year graduate student in the physics department, is an experimental high-energy physicist who is working on one of the most exciting experiments in the world: the muon g-2 experiment. “It’s an enormous, international effort involving more than 200 scientists from 35 institutions. We’re trying to make one of the most precise measurements of all time,” Kessler said.

In a nutshell, the experiment is measuring the g-factor of muons, which are subatomic particles that occur naturally when cosmic rays strike the Earth’s atmosphere. Muons act as if they have a tiny internal magnet inside of them, and the strength of this magnet can be measured and described—this is the muon’s g-factor. The g-factor is not a property of muons alone; it is affected by everything else in the universe that muons interact with. Theoretical physicists have calculated what the muon’s g-factor should be, but experimental data shows a different number. This means that there could be some as-of-yet undiscovered force in the universe that is affecting the muon—and this is where Kessler comes in.

In order to measure these undiscovered forces, whose effects on the muon’s g-factor which are mind-bogglingly small, you first need a ruler of almost unimaginable precision. “My work,” says Kessler, “is calibrating the most precise possible yardstick so that we can accurately measure the muon as precisely as possible.”

David Kawall, Kessler’s advisor, said that “some uncertainties in the experiment that limited the precision of our measurement of the g-factor had proven nearly impossible to overcome” until “Kessler developed an innovative and successful new approach.”

“In the past few months,” Kawall continued, “Kessler made significant further improvements, and the results from his measurements in 2021 are beyond what I ever thought was possible. This is tremendous impact for a graduate student in a large collaboration.”

Kessler will be heading to the Fermilab, in Illinois, to work with their particle accelerator—one of the key instruments in the muon g-2 experiment—for his DOE award.

“Working with Prof. Kawall has been incredible,” said Kessler, “and I’m really excited for the great opportunity to further my research at Fermilab, thanks to the DOE.”

Looking for something that shouldn’t exist

“My research,” says Tristan Winick, also a fifth-year graduate student, “is probing one of the universe’s fundamental mysteries: why is there so much more matter than antimatter?”

The question is an important one because, according to the laws of physics, there should be a nearly symmetrical relationship between matter and antimatter—they should roughly balance each other out. But they don’t, and though the current best physical models can account for some of the imbalance, they can’t account for all of it. This imbalance signifies that there is something happening in the universe that the laws of physics don’t quite fit, given our current knowledge.

What would help explain the matter/antimatter asymmetry is if there is a violation of the “time-reversal symmetry.” The time-reversal symmetry holds that the laws of physics should work the same way whether time runs forwards, as we experience it, or backward. If there’s a violation—if physics worked one way when time moves forward, but in a different way when time runs in reverse—then it could help explain why there is so much more matter than antimatter.

To search for such a time-reversal violation, Winick is working on the Cold Molecule Nuclear Time Reversal Experiment (CeNTREX), which is a collaborative effort between scientists at UMass Amherst, Columbia and Yale. The ultimate goal of CeNTREX is to set a new best limit on the nuclear Schiff moment in a thallium fluoride molecule.

The Schiff moment is when an atom or molecule acts like a magnet. Magnets produce a field with a north and south pole—they are dipolar. Similarly, when an atom or molecule generates a permanent dipolar field, it is having a Schiff moment.

“Nominally, we would not expect the thallium nucleus in thallium fluoride to have a Schiff moment because thallium fluoride is a neutral molecule,” says Winick, “one where all of the positive and negative charges balance out. But, if we do observe one, it would imply a new source of time-reversal violation. That time-reversal violation could account for some of the observed matter-antimatter imbalance in our universe.”

This is a very difficult problem says his advisor, Kawall. “He has overcome many tremendous technical hurdles. It’s a great testament to his persistence and creativity.”

“It’s exciting to be working on one of the fundamental questions in physics,” says Winick, who will be spending the year at the Argonne National Lab, also in Illinois.