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• Physics 16, 139
The Muon g-2 Collaboration has doubled the precision of their 2021 measurement of the muon’s magnetic second, strengthening a stress with predictions primarily based on the usual mannequin.
M. Wynne
Stress was within the air on July 24 as members of the Muon g-2 Collaboration gathered in a convention room on the College of Liverpool, UK. The scientists had congregated to “unblind” their newest measurements of an anomalous property of the muon, an unstable particle that makes up a lot of the cosmic radiation that reaches Earth’s floor. When enter into the pc, would the figures in two sealed envelopes reveal a match between the brand new evaluation and a earlier one? Or would the crew should announce that one thing had gone awry? There was a concern that values could be inconsistent with the collaboration’s earlier findings, says René Reimann, a physicist from the Johannes Gutenberg College Mainz, Germany, who was within the room that day.
Blinding outcomes is “the suitable scientific factor to do, however it’s so nerve wracking,” says James Mott of Fermi Nationwide Accelerator Laboratory (Fermilab), Illinois, who learn out the numbers that day. “There is just one manner it goes proper…and some ways it could possibly go improper.” These nerves rapidly dispelled as Mott learn out the numbers. Not solely have been the values per the older ones, however they have been additionally twice as exact. Mott and Reimann felt aid wash over them. “Everyone was joyful,” Reimann says. “Every part match collectively.”
The numbers shared two weeks in the past in Liverpool—and as we speak with the world—are the most recent replace in a decades-long effort to tease out the precise values of the muon’s magnetic second and its “g-factor” (see Particular Function: The Muon g-2 Anomaly Defined). The magnetic second of a particle—a measure of the torque exerted on it by a magnetic area—is proportional to the particle’s cost and spin through a dimensionless parameter known as the g-factor.
R. Hahn
In a hypothetical world the place the muon behaves as an remoted, idealized level particle, its g-factor will equal 2. In the true world, the place the muon consistently interacts with different “digital” particles flashing out and in of existence, its g-factor is barely bigger than 2. The distinction between actual and hypothetical values—the so-called g 2 anomaly—arises as a result of the digital particles modify the efficient cost of the muon and the velocity at which its spin rotates in a magnetic area. The Muon g-2 experiments measure this velocity and use it to find out the muon’s magnetic second and the g 2 anomaly.
Calculations primarily based on the usual mannequin of particle physics at present predict that the muon has a g-factor of two.00233183620(86). (The numbers in parentheses symbolize the uncertainties within the predictions.) Experiments have lengthy turned up a barely totally different quantity. However it wasn’t till two years in the past that researchers had compelling proof that the discrepancy was actual and never a measurement artifact (see Viewpoint: Muon’s Escalating Problem to the Commonplace Mannequin). That proof got here from experiments by the Muon g-2 Collaboration, which have been carried out at Fermilab (see Analysis Information: Measuring the Magnet that Measures the Muon).
In 2021, the collaboration reported that their measurements—mixed with earlier ones—gave the muon a g-factor of two.00233184122(82), a price that disagrees with predictions. On the time, the experiment–concept discrepancy had a statistical significance of 4.2 sigma, that means that probability that it wasn’t actual was 1 in 40,000. The experimental worth introduced as we speak of two.00233184110(48) is twice as exact because the 2021 determine and the statistical significance of the discrepancy has jumped to five sigma or, if the sooner outcomes are averaged along with the brand new ones, 5.2 sigma. “[Our result] may be very thrilling…and a giant achievement,” says Anna Driutti, a Muon g-2 Collaboration member who works on the College of Pisa in Italy. We have now the “most exact experimental measurement of the muon g 2 up to now, that’s for certain.”
A significance stage of 5 sigma or extra is often sufficient for scientists to say a discovery. However the Muon g-2 Collaboration is cautious on that entrance. Whereas the experimental values now appear established—though their precision is anticipated to extend as soon as the crew finishes its evaluation of one other three years of collected knowledge—the theoretical values will not be but set in stone. In 2021 “lattice-QCD” predictions decided parameters that sit barely nearer to the experimental ones than these obtained with different strategies. Theorists are nonetheless engaged on understanding this discrepancy. “With the brand new uncertainty now standing at 0.20 [parts per million], we’re actually laying a marker all the way down to spur on the idea neighborhood to solidify the prediction,” Mott says.
“A exact measurement of a basic fixed is a permanent consequence that can proceed to yield dividends into the longer term,” says Priscilla Cushman, a physicist on the College of Minnesota who labored on an earlier g-2 experiment. “Whereas we could have to attend till the idea will get sorted out to have faith within the measurement of the standard-model discrepancy, the g 2 measurement marketing campaign at Fermilab has yielded the gold customary by which all newly proposed particles and interactions have to be judged.” She notes that whether or not scientists are proposing new darkish matter candidates or attempting to know anomalous outcomes from ultrahigh-energy particle collisions, reminiscent of these carried out on the Giant Hadron Collider, the fashions they use have to be per the boundaries imposed by g 2.
For the Muon g-2 crew, these issues are for one more day. In the present day, it’s time for them to rejoice. “All of the years of taking extra knowledge and the large efforts of largely early-career researchers in beating down the uncertainties [have] actually paid off,” Mott says. “[We should] be very happy with the achievement.”
–Katherine Wright
Katherine Wright is the Deputy Editor of Physics Journal.
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