The long-awaited first outcomes from the Muon g-2 experiment on the U.S. Division of Power’s Fermi Nationwide Accelerator Laboratory present elementary particles referred to as muons behaving in a manner that’s not predicted by scientists’ greatest idea, the Customary Mannequin of particle physics. This landmark end result, made with unprecedented precision, confirms a discrepancy that has been gnawing at researchers for many years.
The sturdy proof that muons deviate from the Customary Mannequin calculation would possibly trace at thrilling new physics. Muons act as a window into the subatomic world and might be interacting with but undiscovered particles or forces.
“As we speak is a unprecedented day, lengthy awaited not solely by us however by the entire worldwide physics neighborhood,” stated Graziano Venanzoni, co-spokesperson of the Muon g-2 experiment and physicist on the Italian Nationwide Institute for Nuclear Physics. “A considerable amount of credit score goes to our younger researchers who, with their expertise, concepts and enthusiasm, have allowed us to realize this unbelievable end result.”
A muon is about 200 occasions as large as its cousin, the electron. Muons happen naturally when cosmic rays strike Earth’s ambiance, and particle accelerators at Fermilab can produce them in massive numbers. Like electrons, muons act as if they’ve a tiny inside magnet. In a robust magnetic area, the course of the muon’s magnet precesses, or wobbles, very similar to the axis of a spinning high or gyroscope. The power of the inner magnet determines the speed that the muon precesses in an exterior magnetic area and is described by a quantity that physicists name the g-factor. This quantity might be calculated with ultra-high precision.
Because the muons flow into within the Muon g-2 magnet, in addition they work together with a quantum foam of subatomic particles popping out and in of existence. Interactions with these short-lived particles have an effect on the worth of the g-factor, inflicting the muons’ precession to hurry up or decelerate very barely. The Standard Model predicts this so-called anomalous magnetic second extraordinarily exactly. But when the quantum foam comprises further forces or particles not accounted for by the Customary Mannequin, that may tweak the muon g-factor additional.
“This amount we measure displays the interactions of the muon with every part else within the universe. However when the theorists calculate the same amount, utilizing the entire identified forces and particles within the Customary Mannequin, we don’t get the identical reply,” stated Renee Fatemi, a physicist on the College of Kentucky and the simulations supervisor for the Muon g-2 experiment. “That is sturdy proof that the muon is delicate to one thing that’s not in our greatest idea.”
The predecessor experiment at DOE’s Brookhaven Nationwide Laboratory, which concluded in 2001, provided hints that the muon’s conduct disagreed with the Customary Mannequin. The brand new measurement from the Muon g-2 experiment at Fermilab strongly agrees with the worth discovered at Brookhaven and diverges from idea with probably the most exact measurement to this point.
The accepted theoretical values for the muon are:
anomalous magnetic second: 0.00116591810(43)
[uncertainty in parentheses]
The brand new experimental world-average outcomes introduced by the Muon g-2 collaboration right now are:
anomalous magnetic second: 0.00116592061(41)
The mixed outcomes from Fermilab and Brookhaven present a distinction with idea at a significance of 4.2 sigma, somewhat shy of the 5 sigma (or commonplace deviations) that scientists require to say a discovery however nonetheless compelling proof of latest physics. The prospect that the outcomes are a statistical fluctuation is about 1 in 40,000.
The Fermilab experiment reuses the principle part from the Brookhaven experiment, a 50-foot-diameter superconducting magnetic storage ring. In 2013, it was transported 3,200 miles by land and sea from Lengthy Island to the Chicago suburbs, the place scientists may benefit from Fermilab’s particle accelerator and produce probably the most intense beam of muons in the USA. Over the following 4 years, researchers assembled the experiment; tuned and calibrated an extremely uniform magnetic area; developed new methods, instrumentation, and simulations; and completely examined your complete system.
The Muon g-2 experiment sends a beam of muons into the storage ring, the place they flow into 1000’s of occasions at practically the pace of sunshine. Detectors lining the ring permit scientists to find out how briskly the muons are precessing.
In its first 12 months of operation, in 2018, the Fermilab experiment collected extra information than all prior muon g-factor experiments mixed. With greater than 200 scientists from 35 establishments in seven international locations, the Muon g-2 collaboration has now completed analyzing the movement of greater than 8 billion muons from that first run.
“After the 20 years which have handed for the reason that Brookhaven experiment ended, it’s so gratifying to lastly be resolving this thriller,” stated Fermilab scientist Chris Polly, who’s a co-spokesperson for the present experiment and was a lead graduate pupil on the Brookhaven experiment.
Knowledge evaluation on the second and third runs of the experiment is below manner, the fourth run is ongoing, and a fifth run is deliberate. Combining the outcomes from all 5 runs will give scientists an much more exact measurement of the muon’s wobble, revealing with better certainty whether or not new physics is hiding inside the quantum foam.
“Up to now we’ve analyzed lower than 6% of the information that the experiment will finally acquire. Though these first outcomes are telling us that there’s an intriguing distinction with the Customary Mannequin, we are going to study way more within the subsequent couple of years,” Polly stated.
“Pinning down the refined conduct of muons is a outstanding achievement that may information the seek for physics past the Customary Mannequin for years to return,” stated Fermilab Deputy Director of Analysis Joe Lykken. “That is an thrilling time for particle physics analysis, and Fermilab is on the forefront.”
Reference: “Measurement of the Constructive Muon Anomalous Magnetic Second to 0.46 ppm” by B. Abi et al. (Muon g−2 Collaboration), 7 April 2021, Bodily Evaluation Letters.
The Muon g-2 experiment is supported by the Division of Power (US); Nationwide Science Basis (US); Istituto Nazionale di Fisica Nucleare (Italy); Science and Know-how Amenities Council (UK); Royal Society (UK); European Union’s Horizon 2020; Nationwide Pure Science Basis of China; MSIP, NRF and IBS-R017-D1 (Republic of Korea); and German Analysis Basis (DFG).
Fermilab is America’s premier nationwide laboratory for particle physics analysis. A U.S. Division of Power Workplace of Science laboratory, Fermilab is positioned close to Chicago, Illinois, and operated below contract by the Fermi Analysis Alliance LLC.
The DOE Workplace of Science is the one largest supporter of primary analysis within the bodily sciences in the USA and is working to deal with a few of the most urgent challenges of our time.
[Editor’s Note: Today a different group of researchers announced very different results, concluding that the muon’s magnetic field aligns with the standard model of particle physics.]