This image shows the magnetic storage ring at Fermilab for the Muon g-2 experiment. Scientists Zani Semovski, Anna Driutti, Matt Bressler and Fatima Rodriguez can be seen working on the experiment.Ryan Postel/Fermilab
A particle physics experiment decades in the making - the Muon g-2 experiment - looks increasingly like it might set up a showdown over whether there are fundamental particles or forces in the universe that are unaccounted for in the current Standard Model, the comprehensive theory that physicists use to describe how the universe works at its most fundamental level.
On Aug. 10, the international team of scientists behind Muon g-2 - pronounced "g minus 2" - released the world's most precise measurement yet of the anomalous magnetic moment of the muon. Muons are subatomic particles similar to electrons, but about 200 times more massive. Calculating the muon's magnetic moment at a high precision will indicate whether it is interacting solely with the particles and forces known today, or if unknown particles or forces are out there.
"The result we released today confirms earlier findings, but at a much higher level of precision," said David Hertzog, a University of Washington professor of physics and director of the UW Center for Experimental Nuclear Physics and Astrophysics. "Further on, we still have 75% of the data to analyze and already we have exceeded our uncertainty goals. The experiment has been extremely successful".
The Muon g-2 experiment is based at the Fermi National Accelerator Laboratory, a Department of Energy facility near Chicago. At last count, the team includes more than 180 scientists at 33 institutions in seven countries.
Researchers with the UW Precision Muon Physics Group have been part of the Muon g-2 team from the beginning, designing and constructing detectors as well as leading efforts to analyze the massive amounts of data collected. In addition to Hertzog, other UW scientists involved in the team's latest efforts include Peter Kammel, research professor of physics; Erik Swanson, a research engineer with CENPA; and current and former postdoctoral researchers Svende Braun, Christine Claessens, Jarek Kaspar and Zach Hodge. Hertzog noted that seven UW doctoral students, including recent graduates Brynn MacCoy and Hannah Binney, based their dissertations on this experiment. An eighth UW doctoral degree from muon endeavors is forthcoming from Joshua Labounty.
"Working on Muon g-2 has been incredibly exciting," said Labounty. "As we continue to push toward a higher-precision measurement, we've encountered new puzzles, which required a great deal of out-of-the-box thinking. Even processing the petabytes of data necessary for this result has been a challenge."
The collaboration's new measurement for the muon's anomalous magnetic moment is twice as precise as a previous measurement released by the team in 2021. Their findings have been submitted to the journal Physical Review Letters.
By comparing theories built using the Standard Model to experimental results, physicists have been trying to discern whether the theory is complete - that is, whether all particles and forces are known - or if there is physics "beyond the Standard Model." Muons have been playing an increasing role in the gentle tug-of-war between theorists and experimentalists.
Due to the large amount of additional data that is going into the 2023 analysis announcement, the Muon g-2 collaboration's latest result is more than twice as precise as the first result announced in 2021.Muon g-2 collaboration
Like their less massive cousin, the electron, muons have a tiny internal magnet that, in a magnetic field, precesses, or wobbles, like the axis of a spinning top. The precession speed depends on the muon's magnetic moment, typically represented by the letter g; at the simplest level, theory predicts that g should equal 2. Any difference of g from 2 - or "g minus 2" - could be attributed to the muon's interactions with particles blinking in and out of existence in a quantum foam that surrounds it.
The Standard Model predicts how the quantum foam should change g based on the electromagnetic, weak nuclear and strong nuclear forces, as well as photons, electrons, quarks, gluons, neutrinos, W and Z bosons, and the Higgs boson. But physicists are excited about the possible existence of as-yet-undiscovered particles or forces that could contribute to the value of g-2 - and would open the window to exploring new physics.
The new result, based on the first three years of data collected at the team's experimental set-up at Fermilab, is:
g-2 = 0.00233184110 +/- 0.00000000043 (stat.) +/- 0.00000000019 (syst.)
The first number is the calculation, and the second and third are statistical and systemic uncertainties, respectively.
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 muons circulated about 1,000 times at nearly the speed of light. Detectors lining the inside of the ring - including UW-designed and built NMR probes to measure the magnetic field and calorimeters to reconstruct the decay positrons - helped them determine how rapidly the muons were precessing.
The Fermilab experiment reused a storage ring originally built for a predecessor experiment at Brookhaven National Laboratory that concluded in 2001. Officials carefully transported the storage ring 3,200 miles from Long Island to Fermilab. The Muon g-2 experiment, which included improved techniques, instrumentation and simulations, collected data for six years before shutting down the muon beam on July 9, 2023 with a dataset more than 21 times the size of the Brookhaven's.
This new measurement of g-2, which comes from analyzing the first three years of data, corresponds to a precision of 0.20 parts per million.
In 2020, the Muon g-2 Theory Initiative, a related group, announced its best Standard Model prediction for muon g-2 based on data available at the time. A newer experimental measurement of the data, as well as a calculation based on a different theoretical approach, are in tension with the 2020 calculation. The initiative aims to have a new, improved prediction available in the next couple of years that considers both theoretical approaches.
That should come right around the time that the Muon g-2 collaboration anticipates releasing its final, most precise measurement of the muon magnetic moment - setting up an 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 the endeavor's final physics goal.
