Breaking Physics: Muon G-2 Experiment Reinforces Surprise Result, Setting Up ‘Ultimate Showdown’

Findings at Fermilab show discrepancies between theory and experiment, which may lead to new physics beyond the Standard Model.

The physicists now have a new measurement of a property of the muon called anomalous magnetic moment that improves the accuracy of their previous result by a factor of 2.

An international collaboration of scientists working on the Muon g-2 experiment at the US Department of Energy’s Fermi National Accelerator Laboratory. Announced the long-awaited updated measure on August 10. This new value reinforces the first result They announced in April 2021 and establish a confrontation between theory and experiment for more than 20 years.

“We are really exploring new territory. We are determining the magnetic moment of the muon with better precision than ever before,” said Brendan Casey, a senior scientist at Fermilab who has worked on the Muon g-2 experiment since 2008.

Beyond the standard model

Physicists describe how the universe works at its most fundamental level with a theory known as the standard model. By making predictions based on the Standard Model and comparing them to experimental results, physicists can discern whether the theory is complete or whether there is physics beyond the Standard Model.

Muons are fundamental particles that are similar to electrons but about 200 times more massive. Like electrons, muons have a tiny internal magnet that, in the presence of a magnetic field, precesses or wobbles like the axis of a top. The rate of precession in a given magnetic field depends on the magnetic moment of the muon, typically represented by the letter gram; At the simplest level, the theory predicts that gram must be equal to 2.

This seven-minute video provides additional information about muons and the new result of the Muon g-2 collaboration.

The importance of g-2

The difference of gram of 2 – or gram minus 2: can be attributed to the interactions of the muon with the particles in a quantum foam that surrounds it. These particles flicker in and out of existence and, like subatomic “dance partners,” take the muon’s “hand” and change the way the muon interacts with the magnetic field. The Standard Model incorporates all known “dancing partner” particles and predicts how quantum foam changes gram. But there could be more. Physicists are excited about the possible existence of as yet undiscovered particles that contribute to the value of g-2 – and would open the window to explore new physics.

Gordan Krnjaic, a theoretical particle physicist at Fermilab and the University of Chicago Kavli Institute for Cosmological Physics, told the New York Times that if experimental disagreement with the theory persisted, it would be “the first irrefutable laboratory proof of the new physics. And it might well be the first time we’ve broken the standard model.”

Measurement Uncertainties

The new experimental result, based on the first three years of data, announced by the Muon g-2 collaboration is:
g-2 = 0.00233184110 +/- 0.00000000043 (est.) +/- 0.00000000019 (syst.)

The g-2 measurement corresponds to a precision of 0.20 parts per million. The Muon g-2 collaboration describes the result in a document they sent to Physical Review Letters.

With this measure, the collaboration has already achieved its goal of decreasing a particular type of uncertainty: uncertainty caused by experimental imperfections, known as systematic uncertainties.

“This measurement is an incredible experimental achievement,” said Peter Winter, co-spokesperson for the Muon g-2 collaboration. “Reducing systematic uncertainty to this level is a big deal and something we didn’t expect to achieve so soon.”

Although the total systematic uncertainty has already exceeded the design goal, the broader aspect of uncertainty, statistical uncertainty, depends on the amount of data analysed. The result announced today adds an additional two years of data to his first result. The Fermilab experiment will reach its maximum statistical uncertainty once the scientists incorporate the six years’ worth of data into their analysis, which the collaboration aims to complete over the next two years.

Experiment Details

To make the measurement, the Muon g-2 collaboration repeatedly sent a beam of muons into a 50-foot-diameter superconducting magnetic storage ring, where they cycled about 1,000 times at nearly the speed of light. Detectors lining the ring allowed the scientists to determine how fast the muons were being processed. Physicists must also accurately measure the strength of the magnetic field in order to then determine the value of g-2.

The Fermilab experiment reused a storage ring originally built for the predecessor Muon g-2 experiment at DOE’s Brookhaven National Laboratory that concluded in 2001. In 2013, the collaboration transported the 3,200-mile storage ring from Long Island, New York, to Batavia, Illinois. Over the next four years, the collaboration brought together the experiment with improved techniques, instrumentation, and simulations. The main goal of the Fermilab experiment is to reduce the uncertainty of g-2 by a factor of four compared to the Brookhaven result.

In addition to the larger data set, this latest g-2 measurement is improved by updates to the Fermilab experiment itself.

“It could well be the first time we’ve broken the standard model.”

— Gordan Krnjaic, a scientist at Fermilab and UChicago

Conclusion: Future of the Experiment

“Our new measurement is very exciting because it takes us well beyond the Brookhaven sensitivity,” said Graziano Venanzoni, a professor at the University of Liverpool affiliated with the Italian National Institute for Nuclear Physics, Pisa, and a co-spokesperson for the Muon g-2 experiment. at Fermilab.

In addition to the larger data set, this latest g-2 measurement is improved by updates to the Fermilab experiment itself. “We improved a lot of things between our first year of data collection and our second and third years,” said Casey, who recently ended his tenure as Venanzoni’s co-spokesperson. “We were constantly improving the experiment.”

The experiment “really went full steam ahead” for the last three years of data collection, which ended on July 9, 2023. That’s when the collaboration turned off the muon beam, concluding the experiment after six years of data collection. . . They achieved the goal of collecting a data set that is more than 21 times the size of the Brookhaven data set.

Physicists can calculate the effects of the standard model’s well-known “dance partners” on the g-2 muon with incredible precision. The calculations consider the electromagnetic, weak nuclear, and strong nuclear forces, including photons, electrons, quarks, gluons, neutrinos, W and Z bosons, and the Higgs boson. If the standard model is correct, this ultra-precise prediction should match the experimental measurement.

Calculating the standard model prediction for the g-2 muon is very challenging. In 2020, the Muon g-2 Theory Initiative Announced the best Prediction of the standard model for muon g-2 available at that time. But a new experimental measurement of the data that feeds the prediction and a new calculation based on a different theoretical approach, lattice gauge theory, are in tension with the 2020 calculation. Scientists at the Muon g-Theory Initiative 2 pretend to have a new improved prediction available in the coming years that considers both theoretical approaches.

The Muon g-2 collaboration comprises nearly 200 scientists from 33 institutions in seven countries and includes nearly 40 students who have so far received their PhDs based on their work on the experiment. The collaborators will now spend the next two years analyzing the last three years of data. “We expect another factor of two in accuracy when we’re done,” Venanzoni said.

The collaboration anticipates the release of its final and most precise measurement of the muon’s magnetic moment in 2025, which will establish the ultimate showdown between Standard Model theory and experiment. Until then, physicists have a new and improved g-2 muon measurement that is a significant step toward their ultimate physics goal.

The Muon g-2 collaboration featured this scientific paper for publication.

Here it is the recording of the scientific seminar held on August 10, 2023.

The Muon g-2 experiment is supported by the US Department of Energy; National Science Foundation (USA); National Institute of Nuclear Physics (Italy); Science and Technology Facilities Council (UK); Royal Society (UK); Horizon 2020 of the European Union; National Natural Science Foundation of China; MSIP, NRF and IBS-R017-D1 (Republic of Korea); and the German Research Foundation (DFG).