Fermilab’s Muon g-2 Experiment Finally Gives Particle Physicists a Hint of What Lies Beyond the Standard Model

Fermilab’s Muon g-2 Experiment Finally Gives Particle Physicists a Hint of What Lies Beyond the Standard Model

Since the long-awaited detection of the Higgs boson in 2012, particle physicists have ventured deeper into the subatomic realm to explore beyond the Standard Model of particle physics. In doing so, they hope to confirm the existence of previously unknown particles, the existence of exotic physics, and learn more about how the universe began.

At the Fermi National Accelerator Laboratory (also known as Fermilab), researchers conducted the Muon g-2 experiment, which recently announced the results of their first run. Thanks to the unprecedented precision of their instruments, the Fermilab team found that muons in their experiment did not behave in a manner consistent with the Standard Model, and resolved a discrepancy that had existed for decades.

Experiments with muons began decades ago at the European Organization for Nuclear Research (CERN) and were recently carried out at Brookhaven National Laboratory (BNL) in New York. In 2011, Fermilab took over where the BNL left off and dedicated its powerful accelerators to studying the interactions of short-lived muon particles with a strong magnetic field in a vacuum.


Similar to electrons (but with 200 times more mass), muons occur naturally when cosmic rays hit the Earth’s atmosphere. Another similarity is how muons behave like spinning magnets, the strength of which determines the speed at which they move (rotate) in an external magnetic field (known as the “g-factor”). In the case of muons, their g-factor is slightly larger than 2 (hence the name of the experiment).

The purpose of the muon g-2 experiment is to study the rate of precession of muons while exposed to a strong magnetic field. By measuring their g-factor with an accuracy of 0.14 ppm (parts per million), the researchers hope the Muon g? 2 collaboration, whether their behavior matches the predictions of the Standard Model (SM). If not, this would indicate that there is physics that go beyond the SM and need to be considered.

Graziano Venanzoni, physicist at the Italian National Institute for Nuclear Physics (INFN), is also a co-spokesman for the Muon g-2 experiment. As he announced on April 7, the results during the seminar in which the results of the first run were published did not agree with the predictions of the SM:

“Today is an extraordinary day that not only we, but the entire international physics community have been waiting for a long time. A great merit goes to our young researchers who, with their talent, ideas and enthusiasm, have enabled us to achieve this incredible result. “

The previous experiment at the US Department of Energy’s BNL, which was completed in 2001, provided the first indications that muons do not behave in the way that corresponds to the Standard Model. The first results from Fermilab’s most accurate muon g-2 experiment to date are in strong agreement with the results of the BNL research team. At the heart of both experiments is a 15.25 meter (50 foot) superconducting magnetic storage ring.

The Muon g-2 magnet arrives in Fermilab in 2013. Photo credit: Reidar Hahn, Fermilab

This component was transported to Chicago in 2013, where it was integrated into Fermilab’s particle accelerator to create the most intense muon beam in any laboratory in the United States. This beam is directed into the storage ring, where the muons are accelerated to speeds close to the speed of light. As the muons circulate thousands of times, they interact with the short-lived subatomic particles that are constantly going in and out in a vacuum.

These interactions at the quantum level affect the value of the g-factor, thereby speeding up or slowing down the precession of the muons. This leads to what is known as an “anomalous magnetic dipole moment,” where the effects of interactions contribute to a particle’s magnetic moment. This effect is predicted with extreme precision by the SM, but the presence of additional forces beyond the SM or the particles would have an additional effect.

The results obtained by Fermilab and BNL showed an anomalous magnetic moment different from what SM predicts with a significance of 4.2 sigma. Additionally, the researchers found that there was only a 1 in 40,000 chance that their results were due to statistical fluctuations. Renee Fatemi, a physicist at the University of Kentucky and simulation manager for the Muon g-2 experiment, said:

“This quantity that we measure reflects the interactions of the muon with everything else in the universe. However, if the theorists compute the same quantity using all known forces and particles in the Standard Model, we will not get the same answer. This is strong evidence that the muon is sensitive to something that is not in our best theory. “

“Capturing the subtle behavior of muons is a remarkable achievement that will guide the search for physics beyond the Standard Model for years to come,” added Joe Lykken, Fermilab’s assistant director of research. “This is an exciting time for particle physics research and Fermilab is at the forefront.”

The first result of the Muon g-2 experiment confirms the result of the experiment carried out at BNL two decades ago. Photo credit: Ryan Postel / Fermilab / Muon g-2 collaboration

While these results are slightly less than the 5 sigma standard deviation required to declare a positive result, it is still a strong indication of additional physics. In the meantime, the Fermilab team is busy analyzing the data obtained during the second and third runs of the experiment to see if they might have produced even more convincing results. The fourth run is underway and a fifth is planned for the future.

By combining the results of all five runs, the researchers can measure the muon’s g-factor even more accurately. After decades of research, scientists can finally find out whether additional physics is hidden in the quantum foam that permeates time and space. Fermilab scientist Chris Polly, lead graduate student of the Brookhaven experiment and co-spokesperson for the current experiment, said:

“After the 20 years that have passed since the Brookhaven Experiment ended, it is so gratifying to finally solve this mystery. So far we have analyzed less than 6% of the data that the experiment will ultimately collect. While these initial results show that there is a fascinating difference from the Standard Model, we will learn a lot more over the next few years. “

The Fermilab team’s findings were also published on April 7th in an article published in the Physical Review Letters. The muon g? 2 Collaboration is an international consortium made up of members from research institutes and universities from the USA, Italy, Russia, South Korea and Germany.

Further reading: Fermilab

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