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
Physicists use a theory called the Standard Model to describe how the universe works at its most fundamental level. Scientists from around the world have tested this theory in order to determine if there are other forces or particles we have not yet discovered.
One test involves trying to precisely measure the magnetic moment of a subatomic particle called a muon (pronounced "mew-on"). Muons are similar to electrons, but about 200 times more massive. Precisely measuring the muon's magnetic moment, or the magnetic anomaly, will help scientists understand whether it is interacting solely with known particles and forces, or if unknown particles or forces exist.
Scientists working on the Muon g-2 experiment (pronounced "mew-on gee-minus-two"), released two earlier datasets that attempted to precisely measure the muon's magnetic moment. These datasets had some discrepancy between the measured values and calculated values.
On June 3, the team released the third and final measurement of the muon magnetic anomaly. This result agrees with the published results from 2021 and 2023 but with a much better precision of 127 parts per billion, surpassing the original experimental design goal of 140 parts per billion. This new measurement is also closer to the most-recent average calculated value of the magnetic anomaly. These findings have been submitted to the journal Physical Review Letters.
"Now we've published the final dataset and again we obtained a consistent result, but with even smaller uncertainties," said David Hertzog, a University of Washington professor of physics and director of the UW Center for Experimental Nuclear Physics and Astrophysics. "This is a legacy experiment with extraordinary precision on a very fundamental quantity. It probes the Standard Model in a broad and impactful manner. If these newest results are true, they would suggest no new physics, but it's 'stay tuned' for now because it will take more time for researchers in the theory community to come to their own precise prediction."
The third and final result from the Muon g-2 collaboration, based on the last three years of data, is in perfect agreement with the previous results, further solidifying the experimental world average.Samantha Koch, Fermilab for the Muon g-2 collaboration
The Muon g-2 experiment looks at what's known as "the wobble" of the muon. Like electrons, muons have a quantum mechanical property called "spin" that can be interpreted as a tiny internal magnet. In the presence of an external magnetic field, the internal magnet will wobble - or precess - 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 other particles pulling at the muon's precession.
The Standard Model predicts how g should change based on the electromagnetic, weak nuclear and strong nuclear forces, as well as particles such as photons, electrons, quarks, gluons, neutrinos, W and Z bosons, and the Higgs boson. But if the measured value of g is different from the Standard Model's value, it could suggest the possible existence of as-yet-undiscovered particles or forces that could contribute to the value of g-2 - and this could open the window to exploring new physics.
When measurements taken at Brookhaven National Laboratory in the late 1990s and early 2000s showed a possible discrepancy with the theoretical calculation at that time, physicists decided to upgrade the Muon g-2 experiment to make a more precise measurement.
For this experiment, 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.
This 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 original dataset.
This final measurement is based on the analysis of the last three years of data, taken between 2021 and 2023, combined with the previously published datasets. This more than tripled the size of the dataset used for their second result in 2023, and it enabled the collaboration to finally achieve the precision goal proposed in 2012.
The latest experimental value of the magnetic moment of the muon from the Fermilab experiment is:
g-2 = 0.001165920705 +/- 0.000000000114(stat.) +/- 0.000000000091(syst.)
The first number is the calculation, and the second and third are statistical and systematic uncertainties, respectively.
This dataset also represents an analysis of the experiment's best-quality data. Toward the end of their second data-taking run, the Muon g-2 collaboration finished tweaks and enhancements to the experiment that improved the quality of the muon beam and reduced uncertainties.
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 179 scientists at 37 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 includes Peter Kammel, research professor of physics; Erik Swanson, a research engineer with CENPA; and current and former postdoctoral researchers Jarek Kaspar, Zach Hodge, Svende Braun, Christine Claessens, Brynn MacCoy and Joshua LaBounty. Hertzog noted that seven former UW doctoral students - Rachel Osofsky, Matthias Smith, Nathan Froemming, Aaron Fienberg, Hannah Binney, Brynn MacCoy and Joshua LaBounty - based their dissertations on this experiment.
"Our UW group even spawned three new g-2 groups as postdocs who went on to be professors built their own groups," Hertzog said.
This result will remain the world's most precise measurement of the muon magnetic anomaly for many years to come. Despite recent challenges with the theoretical predictions that reduce evidence of new physics from Muon g-2, this result provides a stringent benchmark for proposed extensions of the Standard Model of particle physics.
"This is a very exciting moment because we not only achieved our goals but exceeded them, which is not very easy for these precision measurements," said Peter Winter, who was a postdoctoral researcher in Hertzog's group and is now a physicist at Argonne National Laboratory and the co-spokesperson for the Muon g-2 collaboration. "With the support of the funding agencies and the host lab, Fermilab, it has been very successful overall, as we reached or surpassed pretty much all the items that we were aiming for."
A plot showing the accumulated amount of data analyzed (in number of positrons) over the six data-taking periods, or runs, from April 2018 to May 2023.Samantha Koch, Fermilab for the Muon g-2 collaboration
While the experiment's main analysis has come to an end, there is more to be mined from the six years of Muon g-2 data. In the future, the collaboration will produce measurements of a property of the muon called the electric dipole moment as well as tests of a fundamental property of physical laws known as charge, parity and time-reversal symmetry.
"Of course, it's sad to end such an endeavor because it's been a large part of many of our collaborators' lives," said Winter, who has been part of the collaboration since 2011. "But we also want to move to the next physics that's out there, to do our best to advance the field in other areas. I think it will be a textbook experiment that will be a long-lasting reference for many future decades to come.
