Long-awaited muon physics experiment nears moment of truth

After a two-decade wait that included an extended wrestle for funding and a transfer midway throughout a continent, a rebooted experiment on the muon — a particle much like the electron however heavier and unstable — is about to unveil its outcomes. Physicists have excessive hopes that its newest measurement of the muon’s magnetism, scheduled to be launched on 7 April, will uphold earlier findings that might result in the invention of new particles.

The Muon g – 2 experiment, now primarily based on 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 authentic outcomes, introduced in 2001 after which finalized in 2006¹, discovered that the muon’s magnetic moment — a measure of the magnetic area it generates — is barely bigger than principle predicted. This prompted a sensation, and spurred controversy, amongst physicists. If these outcomes are finally confirmed — in subsequent week’s announcement, or by future experiments — they might reveal the existence of new elementary particles and upend elementary physics. “Everybody’s antsy,” says Aida El-Khadra, a theoretical physicist on the University of Illinois in Urbana-Champaign.

Magnetic measurements

Muon g – 2 measures the muon’s magnetic moment by transferring the particles round in a 15-metre-diameter circle. A robust magnet retains the muons on their round observe, and on the identical time makes their magnetic north–south axis rotate. The stronger the particles’ magnetic moment, the quicker 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 labored on Muon g – 2 and its predecessor since 1989.

The discrepancy from theoretical expectations that the unique experiment discovered was tiny, however sufficiently big to trigger a stir amongst theoreticians. To first approximation, quantum physics predicts that elementary particles such because the muon and the electron have a magnetic moment precisely equal to 2 (in items of measurement that rely upon the particle). But a fuller calculation reveals a deviation from this excellent worth, brought on by the truth that empty area isn’t actually empty. The area round a muon seethes with all types of ‘virtual particles’ — ephemeral variations of precise particles that constantly seem and disappear from the vacuum — which alter the muon’s magnetic area.

The extra sorts of particle that exist, the extra their digital variations have an effect on the magnetic moment. This implies that a high-precision measurement may reveal oblique proof for the existence of beforehand unknown particles. “Basically what we’re measuring is a number that’s the sum of everything nature has got out there,” says Roberts.

The ensuing magnetic moment is barely barely completely different from 2, and that tiny distinction is often denoted by g – 2. At Brookhaven, the physicists discovered g – 2 to be 0.0023318319. At the time, this was barely bigger than theoreticians’ finest estimates of the contributions from recognized digital particles.

The precision of the measurement was not excessive sufficient to say with confidence that the discrepancy was actual, nevertheless it was giant sufficient to trigger pleasure. The outcomes additionally got here at a time when the sector appeared poised for an explosive interval of discovery. The Large Hadron Collider (LHC) was beneath building on the Swiss–French border, and theorists believed it will uncover scores of new particles. But other than the historic 2012 discovery of the Higgs boson, the LHC has not discovered another elementary particles. Moreover, its knowledge have dominated out many potential candidates for digital particles that might have inflated the muon’s magnetic moment, says Michael Peskin, a theoretical physicist on the SLAC National Accelerator Laboratory in Menlo Park, California.

But the LHC didn’t rule out all attainable explanations for the discrepancy, Peskin says. Among them, says theoretical physicist Dominic Stöckinger on the University of Dresden in Germany, is that there’s not only one sort of Higgs bosons however at the least two.

Evolving principle

At the time of the Brookhaven experiment, the experimental worth for the muon’s magnetic moment needed to be in contrast with theoretical predictions that themselves got here with comparatively giant uncertainties. But whereas the perfect experimental measurement of g – 2 has not modified in 15 years, the speculation has advanced. Last 12 months, a big collaboration co-chaired by El-Khadra introduced collectively a number of groups of researchers — every specializing in a single sort of digital particle — and printed a ‘consensus’ worth for the basic fixed². The discrepancy between theoretical and experimental values didn’t budge.

Also final 12 months, a workforce referred to as the Budapest-Marseille-Wuppertal Collaboration posted a preprint that instructed a theoretical worth for g – 2 nearer to the experimental one³. The workforce targeted on one notably cussed supply of uncertainty within the principle, coming from digital variations of gluons, the particles that transmit the robust nuclear power. If their outcomes are right, the hole between principle and experiment might become non-existent. The preliminary findings, that are at present present process overview for publication, “caused a big splash” and have since been fiercely debated, says El-Khadra.

The outcomes to be unveiled on 7 April may not settle the difficulty fairly but. Thanks to upgrades to the equipment, the workforce finally expects to enhance the accuracy of g – 2 fourfold in contrast with the Brookhaven experiment. But it has to this point analyzed just one 12 months’s price of the info collected since 2017 — not sufficient for the margin of error to be narrower than for the Brookhaven experiment. Still, Roberts says, if the measurement carefully matches the unique one, confidence in that outcome will enhance.

If Fermilab finally confirms the Brookhaven shock, the scientific group will most likely demand an extra, impartial affirmation. That may come from an experimental method being developed on the Japan Proton Accelerator Research Complex (J-PARC) close to Tokai, which might measure the magnetic moment of the muon in a radically completely different manner.

Additional reporting by Elizabeth Gibney.