Following a two-decade wait around that included a extensive wrestle for funding and a move midway across a continent, a rebooted experiment on the muon—a particle very similar to the electron but heavier and unstable—is about to unveil its final results. Physicists have large hopes that its latest measurement of the muon’s magnetism, scheduled to be launched on 7 April, will uphold previously results that could guide to the discovery of new particles.
The Muon g – 2 experiment, now based mostly at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, very first ran between 1997 and 2001 at Brookhaven Nationwide Laboratory on Very long Island, New York. The unique results, declared in 2001 and then finalized in 2006, located that the muon’s magnetic moment—a evaluate of the magnetic subject it generates—is a little bit greater than concept predicted. This caused a sensation, and spurred controversy, amid physicists. If these success are in the end confirmed—in up coming week’s announcement, or by potential experiments—they could expose the existence of new elementary particles and upend basic physics. “Everybody’s antsy,” suggests Aida El-Khadra, a theoretical physicist at the College of Illinois in Urbana-Champaign.
Magnetic measurements
Muon g – 2 measures the muon’s magnetic second by going the particles close to in a 15-metre-diameter circle. A potent magnet keeps the muons on their round observe, and at the exact same time helps make their magnetic north–south axis rotate. The stronger the particles’ magnetic instant, the speedier the axis will spin. “What we measure is the amount at which the muon rotates in the magnetic discipline, like a [spinning] prime that precesses,” states Lee Roberts, a physicist at Boston University in Massachusetts, who has labored on Muon g – 2 and its predecessor given that 1989.
The discrepancy from theoretical expectations that the unique experiment located was little, but massive adequate to induce a stir among theoreticians. To initial approximation, quantum physics predicts that elementary particles this kind of as the muon and the electron have a magnetic second accurately equal to 2 (in models of measurement that count on the particle). But a fuller calculation reveals a deviation from this best worth, induced by the actuality that vacant area is in no way truly empty. The room all around a muon seethes with all varieties of ‘virtual particles’—ephemeral variations of real particles that constantly show up and disappear from the vacuum—which alter the muon’s magnetic subject.
The additional forms of particle that exist, the much more their virtual versions have an affect on the magnetic moment. This usually means that a large-precision measurement could reveal oblique proof for the existence of formerly unfamiliar particles. “Basically what we’re measuring is a amount that is the sum of every thing nature has acquired out there,” claims Roberts.
The ensuing magnetic minute is only slightly various from 2, and that small difference is usually denoted by g – 2. At Brookhaven, the physicists found g – 2 to be .0023318319. At the time, this was somewhat greater than theoreticians’ most effective estimates of the contributions from recognized virtual particles.
The precision of the measurement was not substantial adequate to declare with confidence that the discrepancy was actual, but it was big enough to lead to pleasure. The final results also came at a time when the field appeared poised for an explosive period of time of discovery. The Significant Hadron Collider (LHC) was below construction on the Swiss–French border, and theorists considered it would discover scores of new particles. But aside from the historic 2012 discovery of the Higgs boson, the LHC has not found any other elementary particles. Additionally, its facts have ruled out quite a few possible candidates for virtual particles that could have inflated the muon’s magnetic second, claims Michael Peskin, a theoretical physicist at the SLAC Countrywide Accelerator Laboratory in Menlo Park, California.
But the LHC did not rule out all feasible explanations for the discrepancy, Peskin claims. Among the them, states theoretical physicist Dominik Stöckinger at the College of Dresden in Germany, is that there is not just one particular variety of Higgs bosons but at minimum two.
Evolving concept
At the time of the Brookhaven experiment, the experimental benefit for the muon’s magnetic instant had to be in contrast with theoretical predictions that themselves arrived with somewhat large uncertainties. But while the finest experimental measurement of g – 2 has not changed in 15 many years, the idea has advanced. Last 12 months, a substantial collaboration co-chaired by El-Khadra introduced alongside one another several teams of researchers—each specializing in one kind of virtual particle—and posted a ‘consensus’ value for the elementary frequent. The discrepancy amongst theoretical and experimental values did not budge.
Also final 12 months, a crew identified as the Budapest-Marseille-Wuppertal Collaboration posted a preprint that recommended a theoretical value for g – 2 closer to the experimental just one. The crew concentrated on a single specifically stubborn source of uncertainty in the theory, coming from virtual variations of gluons, the particles that transmit the strong nuclear power. If their benefits are correct, the gap among theory and experiment may well turn out to be non-existent. The preliminary results, which are presently going through evaluation for publication, “caused a large splash” and have since been fiercely debated, says El-Khadra.
The success to be unveiled on 7 April could possibly not settle the issue rather yet. Many thanks to updates to the equipment, the staff eventually expects to boost the precision of g – 2 fourfold when compared with the Brookhaven experiment. But it has so far analysed only a person year’s value of the details collected considering the fact that 2017—not more than enough for the margin of error to be narrower than for the Brookhaven experiment. Nevertheless, Roberts states, if the measurement carefully matches the original a person, self-assurance in that end result will increase.
If Fermilab ultimately confirms the Brookhaven surprise, the scientific neighborhood will possibly demand from customers a additional, independent confirmation. That could appear from an experimental method becoming produced at the Japan Proton Accelerator Analysis Elaborate (J-PARC) around Tokai, which would evaluate the magnetic instant of the muon in a radically unique way.
Added reporting by Elizabeth Gibney.
This short article is reproduced with permission and was initial published on March 30 2021.