New findings take Regis professor and collaborators closer to uncovering new physics

From the campus of Fermilab to his Regis classroom, Fred Gray has spent his career collaborating with scientists around the world to measure the seemingly immeasurable — and potentially uncover particles or forces currently unknown to the world of physics.

Gray, chair of the Regis Department of Physics and Astronomy, and physicists worldwide have been trying to resolve apparent differences between theoretical calculations and actual measurements of a subatomic particle called the muon. In the process, scientists are attempting to determine whether physics goes beyond the Standard Model — the best explanation for how the subatomic world works.

Thursday, Fermi National Accelerator Lab announced the much-anticipated results of a second-round of results in its Muon g-2 experiment, which Gray was part of, that produced an updated and more precise measurement of what physicists call the anomalous magnetic moment of muons.

This new value bolsters the first result announced in April 2021 and sets up “a showdown between theory and experiment over 20 years in the making” according to a statement released by Fermilab. Read the full announcement.

The results of a first round of experiments to gain that measurement showed strong evidence that the theory of physics known as the Standard Model is incomplete. According to the U.S. Department of Energy, the Standard Model explains how particles called quarks (which make up protons and neutrons) and leptons (which include electrons) make up all known matter. 

The Standard Model explains three of the four forces that scientists believe govern the universe: electromagnetism; strong force and weak force. But physicists understand that much of the universe is not made of ordinary matter as we know it, but consists of dark matter and dark energy that do not fit into the Standard Model.

Most of us learn about protons, neutrons and electrons in elementary school. But there are many more subatomic particles moving around in the universe. In the 1930s, muons — which have the same charge as electrons but are about 200 times heavier and have a lifespan of about 2.2 microseconds — became one of the first of those additional particles discovered.

In the Fermilab experiments, focusing magnets help transport the muons into a powerful magnet. The first round of experiments found a 4.2 standard deviation discrepancy when compared with the Standard Model, just short of the 5 standard deviations that would qualify as a discovery in physics. With the new results, scientists are one step closer to achieving discovery status.

“Like electrons, muons have a tiny internal magnet that, in the presence of a magnetic field, precesses or wobbles like the axis of a spinning top. The precession speed in a given magnetic field depends on the muon magnetic moment, typically represented by the letter g; at the simplest level, theory predicts that g should equal 2,” Fermilab officials wrote in a release.

“The difference of g from 2 — or g minus 2 — can be attributed to the muon’s interactions with particles in a quantum foam that surrounds it. These particles blink in and out of existence and, like subatomic ‘dance partners,’ grab the muon’s ‘hand’ and change the way the muon interacts with the magnetic field. The Standard Model incorporates all known ‘dance partner’ particles and predicts how the quantum foam changes g. But there might be more. Physicists are excited about the possible existence of as-yet-undiscovered particles that contribute to the value of g-2 — and would open the window to exploring new physics,” according to Fermilab.

Gray said the latest results reflect a combination of data from the first and second runs of the experiment. Fermilab anticipates releasing the final round of results in 2025, bringing together six years of data collected during the experiments.

During the experiments, Gray dedicated Regis breaks and extra time to traveling to Fermilab, which is outside Chicago. He worked on the experiment even before that, writing his 2003 dissertation on results from a related experiment conducted at Brookhaven National Laboratory.

During the 2021-22 academic year, Gray took a sabbatical from Regis to work as the operations manager of the fifth run of the experiment, overseeing its day-to-day operations. On a typical day, he supervised graduate students and postdoctoral fellows who worked as run coordinators. His work got interesting, he said, when the magnet stopped working. Because the magnet is a complex piece of equipment, it sometimes ran into issues, which Gray helped solve.

Gray also has enlisted his Regis students to help. Their duties have included refurbishing fiber harps, which measure motion. Over the summer, Regis students also accompanied Gray to CERN, the European Organization for Nuclear Research, where he is working on another experiment with the potential to disrupt the Standard Model's prediction for g-2.

In a 2021 interview, before the Muon g-2 initial results were released, Gray said, “It’s unlikely that we would have to throw out the book on particle physics. But it’s more likely we would have to add another chapter, or maybe even another whole volume of the book.”

A month before the newest results were released Gray speculated that in 20 to 30 years, physics might have more answers — but it will take more experiments that measure something anomalous to finally break the Standard Model.

Until then, Gray and his students expect to keep working on Muon g-2 and related experiments, taking further steps toward expanding the world’s understanding of physics.

Photo: The Muon g-2 ring is located at Fermilab in Illinois. Courtesy of Ryan Postel, Fermilab