University Of Massachusetts Amherst Expert Studies The Secrets Of The Universe, With Help From His Students

UMass Amherst’s David Kawall, professor of physics, is part of a large international team of almost 200 physicists exploring the uncharted territory at the edges of the Standard Model, the most comprehensive understanding we have of how everything in the universe works. And he’s doing it with the help of his students.

Physicists describe how the universe works at its most fundamental level with a theory known as the Standard Model. By making predictions based on the Standard Model and comparing them to experimental results, physicists can discern whether the theory is complete — or if there is physics beyond the Standard Model.

“Though the Standard Model is incredibly successful and describes pretty much everything we see,” says Kawall, “we know that it is incomplete, because it can’t account for things like dark matter.”

To fill in the Standard Model’s gaps, an international collaboration of scientists, including Kawall, have been hard at work on the Muon g-2 experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory. They recently made the world’s most precise measurement yet of the muon’s “anomalous magnetic moment”—or the amount that the muon’s internal magnet wobbles in a strong magnetic field—bringing particle physics closer to the ultimate showdown between theory and experiment that may uncover new particles or forces. This new value bolsters the first result they announced in April 2021 and sets up a showdown between theory and experiment over 20 years in the making.

“We’re really probing new territory. We’re determining the muon magnetic moment at a better precision than it has ever been seen before,” said Brendan Casey, a senior scientist at Fermilab who has worked on the Muon g-2 experiment since 2008.

Muons are fundamental particles that are similar to electrons but about 200 times as massive. 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.

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.

David Kawall
It’s very difficult to measure the after-effects of the kicker on the muons so that we could account for it in our final calculations, and Bressler and Kessler were crucial in helping us fine-tune our approach.

David Kawall, professor of physics

To make the measurement, the Muon g-2 collaboration repeatedly sent a beam of muons into a 50-foot-diameter superconducting magnetic storage ring, where they circulated about 1,000 times at nearly the speed of light. Detectors lining the ring allowed scientists to determine how rapidly the muons were precessing. Physicists must also precisely measure the strength of the magnetic field to then determine the value of g-2.

This is where Kawall and his students, including postdoctoral researcher Matthew Bressler, graduate student David Kessler, and undergrads Arthur Alvez and Mor Evron, come in. His group worked on measuring the strength of the magnetic field through which the muons passed, as well as preparing the magnet itself, a feat requiring almost unimaginable precision. The team also spent years developing special calibration probes of incredible fidelity, accurate down to 15 parts per billion.

“One of the major contributions that Bressler and Kessler made is to measurements of the after-effects of the ‘kicker,’” says Kawall. When the muons are injected into the storage ring, their orbits need to be tweaked so that they don’t collide with instrument itself. The kicker tweaks the muons’ orbits with its own magnetic field, which, when you’re trying to measure the magnetic field of the muons themselves, is an obvious source of contamination. “It’s very difficult to measure the after-effects of the kicker on the muons so that we could account for it in our final calculations,” says Kawall, “and Bressler and Kessler were crucial in helping us fine-tune our approach.”

Together, the international team has determined that g-2 = 0.00233184110 +/- 0.00000000043 (statistical error) +/- 0.00000000019 (systematic error)

“This measurement is an incredible experimental achievement,” said Peter Winter, co-spokesperson for the Muon g-2 collaboration. “Getting the systematic uncertainty down to this level is a big deal and is something we didn’t expect to achieve so soon.”

While the total systematic uncertainty has already surpassed the design goal, the larger aspect of uncertainty — statistical uncertainty — is driven by the amount of data analyzed. The result announced today adds an additional two years of data to their first result. The Fermilab experiment will reach its ultimate statistical uncertainty once scientists incorporate all six years of data in their analysis, which the collaboration aims to complete in the next couple of years.

The Muon g-2 collaboration comprises close to 200 scientists from 33 institutions in seven countries and includes nearly 40 students so far who have received their doctorates based on their work on the experiment. Collaborators will now spend the next couple of years analyzing the final three years of data. “We expect another factor of two in precision when we finish,” said Venanzoni.

The collaboration anticipates releasing their final, most precise measurement of the muon magnetic moment in 2025 — setting up the ultimate showdown between Standard Model theory and experiment. Until then, physicists have a new and improved measurement of muon g-2 that is a significant step toward its final physics goal.