Long-awaited muon physics experiment nears moment of truth

After a two-decade wait that included a long struggle for funding and a move across the continent, a restarted experiment on the muon – a particle similar to the electron but heavier and unstable – is about to be completed. unveil its results. Physicists have high hopes that its latest muon magnetism measurement, scheduled for April 7, will confirm 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 took place between 1997 and 2001 at the Brookhaven National Laboratory in Long Island, New York. The original results, announced in 2001 then finalized in 2006¹, found that the muon’s magnetic moment – a measure of the magnetic field it generates – is slightly larger than theory predicted. It caused a sensation and aroused controversy among physicists. If these results are finally confirmed – in next week’s announcement or by future experiments – they could reveal the existence of new elementary particles and disrupt fundamental physics. “Everyone is anxious,” says Aida El-Khadra, theoretical physicist at the University of Illinois at Urbana-Champaign.

Magnetic measurements

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

The deviation from theoretical expectations that the original experiment found was minimal, but large enough to cause a stir among theorists. As a first approximation, quantum physics predicts that elementary particles like the muon and the electron have a magnetic moment exactly equal to 2 (in units of measurement which depend on the particle). But a more complete calculation reveals a deviation from this perfect value, caused by the fact that empty space is never really empty. The space around a muon is bubbling with all kinds of “virtual particles” – ephemeral versions of real particles that continually appear and disappear from a vacuum – which alter the muon’s magnetic field.

The more types of particles there are, 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 is the sum of everything nature has,” says Roberts.

The resulting magnetic moment is only slightly different from 2, and this small difference is usually referred to as g – 2. At Brookhaven, physicists found g – 2 is 0.0023318319. At the time, this figure was slightly higher than theorists’ best estimates of the contributions of known virtual particles.

The accuracy of the measurement was not high enough to say for sure that the deviation was real, but it was large enough to cause excitement. The results also came at a time when the field looked set for a period of explosive discovery. The Large Hadron Collider (LHC) was under construction on the Franco-Swiss border and theorists believed it would discover dozens of new particles. But apart from the historic discovery of the Higgs boson in 2012, the LHC has not found any other elementary particles. What’s more, its data ruled out many potential candidates for virtual particles that could have inflated the muon’s magnetic moment, says Michael Peskin, theoretical physicist at SLAC National Accelerator Laboratory in Menlo Park, California.

But the LHC hasn’t ruled out all possible explanations for the gap, Peskin says. Among them, says theoretical physicist Dominik Stöckinger of the University of Dresden in Germany, there is not just one type of Higgs boson but at least two.

Evolving theory

At the time of the Brookhaven experiment, the experimental value of the muon’s magnetic moment had to be compared with theoretical predictions which themselves were accompanied by relatively large uncertainties. But while the best experimental measure 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 constant². The difference between theoretical and experimental values ​​has not changed.

Also last year, a team called the Budapest-Marseille-Wuppertal Collaboration published a preprint suggesting a theoretical value for g – 2 closer to the experimental³. The team focused on a particularly stubborn source of uncertainty in the theory, stemming from virtual versions of gluons, the particles that transmit strong nuclear force. If their results are correct, the gap between theory and experience could turn out to be nonexistent. The preliminary results, which are currently being reviewed for publication, “caused a stir” and have since been hotly debated, El-Khadra said.

The results, which will be released on April 7, may not yet fully resolve the issue. Through device upgrades, the team ultimately hopes to improve the accuracy of g – 2 quadruple compared to the Brookhaven experiment. But so far, he’s only analyzed a year’s worth of data collected since 2017 – not enough for the margin of error to be narrower than for Brookhaven’s experiment. Still, says Roberts, if the metric closely matches the original one, confidence in that result will improve.

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

Additional reporting by Elizabeth Gibney.