Long-Awaited Muon Physics Experiment Nears Moment of Truth

After a two-decade wait that included a long struggle for funding and a move halfway across a continent, a rebooted experiment on the muon—a particle similar to the electron but heavier and unstable—is about to unveil its results. Physicists have high hopes that its latest measurement of the muon’s magnetism, scheduled to be released on 7 April, will uphold earlier findings that could lead to the discovery of new particles.

The Muon g – 2 experiment, now based at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, first ran between 1997 and 2001 at Brookhaven National Laboratory on Long Island, New York. The original results, announced in 2001 and then finalized in 2006, found that the muon’s magnetic moment—a measure of the magnetic field it generates—is slightly larger than theory predicted. This caused a sensation, and spurred controversy, among physicists. If those results are ultimately confirmed—in next week’s announcement, or by future experiments—they could reveal the existence of new elementary particles and upend fundamental physics. “Everybody’s antsy,” says Aida El-Khadra, a theoretical physicist at the University of Illinois in Urbana-Champaign.

Magnetic measurements

Muon g – 2 measures the muon’s magnetic moment by moving the particles around in a 15-metre-diameter circle. A powerful magnet keeps the muons on their circular track, and at the same time makes their magnetic north–south axis rotate. The stronger the particles’ magnetic moment, the faster the axis will spin. “What we measure is the rate at which the muon rotates in the magnetic field, like a [spinning] top that precesses,” says Lee Roberts, a physicist at Boston University in Massachusetts, who has worked on Muon g – 2 and its predecessor since 1989.

The discrepancy from theoretical expectations that the original experiment found was tiny, but big enough to cause a stir among theoreticians. To first approximation, quantum physics predicts that elementary particles such as the muon and the electron have a magnetic moment exactly equal to 2 (in units of measurement that depend on the particle). But a fuller calculation reveals a deviation from this perfect value, caused by the fact that empty space is never truly empty. The space around a muon seethes with all kinds of ‘virtual particles’—ephemeral versions of actual particles that continuously appear and disappear from the vacuum—which alter the muon's magnetic field.

The more types of particle that exist, the more their virtual versions affect the magnetic moment. This means that a high-precision measurement could reveal indirect evidence for the existence of previously unknown particles. “Basically what we’re measuring is a number that’s the sum of everything nature has got out there,” says Roberts.

The resulting magnetic moment is only slightly different from 2, and that tiny difference is commonly denoted by g – 2. At Brookhaven, the physicists found g – 2 to be 0.0023318319. At the time, this was slightly larger than theoreticians’ best estimates of the contributions from known virtual particles.

The precision of the measurement was not high enough to claim with confidence that the discrepancy was real, but it was large enough to cause excitement. The results also came at a time when the field seemed poised for an explosive period of discovery. The Large Hadron Collider (LHC) was under construction on the Swiss–French border, and theorists believed it would discover scores of new particles. But apart from the historic 2012 discovery of the Higgs boson, the LHC has not found any other elementary particles. Moreover, its data have ruled out many potential candidates for virtual particles that could have inflated the muon’s magnetic moment, says Michael Peskin, a theoretical physicist at the SLAC National Accelerator Laboratory in Menlo Park, California.

But the LHC did not rule out all possible explanations for the discrepancy, Peskin says. Among them, says theoretical physicist Dominik Stöckinger at the University of Dresden in Germany, is that there is not just one type of Higgs bosons but at least two.

Evolving theory

At the time of the Brookhaven experiment, the experimental value for the muon’s magnetic moment had to be compared with theoretical predictions that themselves came with relatively large uncertainties. But whereas the best experimental measurement of g – 2 has not changed in 15 years, the theory has evolved. Last year, a large collaboration co-chaired by El-Khadra brought together several teams of researchers—each specializing in one type of virtual particle—and published a ‘consensus’ value for the fundamental constant. The discrepancy between theoretical and experimental values did not budge.

Also last year, a team called the Budapest-Marseille-Wuppertal Collaboration posted a preprint that suggested a theoretical value for g – 2 closer to the experimental one. The team focused on one particularly stubborn source of uncertainty in the theory, coming from virtual versions of gluons, the particles that transmit the strong nuclear force. If their results are correct, the gap between theory and experiment might turn out to be non-existent. The preliminary findings, which are currently undergoing review for publication, “caused a big splash” and have since been fiercely debated, says El-Khadra.

The results to be unveiled on 7 April might not settle the issue quite yet. Thanks to upgrades to the apparatus, the team ultimately expects to improve the accuracy of g – 2 fourfold compared with the Brookhaven experiment. But it has so far analysed only one year’s worth of the data collected since 2017—not enough for the margin of error to be narrower than for the Brookhaven experiment. Still, Roberts says, if the measurement closely matches the original one, confidence in that result will improve.

If Fermilab ultimately confirms the Brookhaven surprise, the scientific community will probably demand a further, independent confirmation. That could come from an experimental technique being developed at the Japan Proton Accelerator Research Complex (J-PARC) near Tokai, which would measure the magnetic moment of the muon in a radically different way.

Additional reporting by Elizabeth Gibney.

This article is reproduced with permission and was first published on March 30 2021.