Signs of new physics after a tiny particle wobble

This Wednesday the expected first results of the experiment were presented Muon g-2 at the Fermi National Laboratory (Fermilab) of the US Department of Energy and have surprised by showing that fundamental particles called muons behave in a way that does not predict the best of the theories in this field: the standard model of the physics of particles.

This could hint that there is an exciting new physics unknown until now. Muons act as a window to the subatomic world and could be interacting with particles or forces that have yet to be discovered. So far the gravitational, electromagnetic, strong and weak forces are known, but there could be a fifth.

The first results of the Muon g-2 experiment by Fermilab (USA) show that muons behave in a way that is not predicted by the standard model of physics, so they could be interacting with particles or forces unknown to science

This momentous result, also published in the journal Physical Review Letters, has been made with unprecedented precision, and confirms a discrepancy that has intrigued researchers for decades.

“Today is an extraordinary day, long awaited not only by us, but by the entire international physical community,” he highlighted. Graziano Venanzoni, co-spokesperson for the Muon g-2 experiment and physicist at the National Institute of Nuclear Physics in Italy, who has highlighted the important role of young researchers in this finding.

The wobble of the muon, the ‘cousin’ of the electron

A muon is about 200 times more massive than its ‘cousin’, the electron. Muons are produced naturally when cosmic rays strike Earth’s atmosphere, and Fermilab’s particle accelerators can produce them in large quantities. Like electrons, they act as if they have a small internal magnet.

In a strong magnetic field, the direction of the muon magnet precesses, wobbles like the axis of a spinning top. The force of the internal magnet determines the rate of precession of this particle in an external magnetic field and is described by a number that physicists call g factor. This number can be calculated with extremely high precision.

The new data from Fermilab coincide with those obtained two decades ago at Brookhaven National Laboratory, diverging from the theory with the most precise measurement made to date.

As the muons circulate through the Muon g-2 magnet, they also interact with a quantum foam of subatomic particles that appear and disappear. Interactions with these short-lived particles affect the value of the g factor, causing the precession of the muons to speed up or slow down very slightly. The Standard Model predicts this so-called anomalous magnetic moment with extreme precision. But if the quantum foam contains additional forces or particles that are not contemplated in the model, that would further modify the g-factor of the muons, and this appears to have happened.

“This quantity that we measure reflects the interactions of the muon with everything else in the universe, but when theorists calculate that same quantity, using all known forces and particles from the standard model, we don’t get the same answer,” he explains. Renee Fatemi, a physicist at the University of Kentucky and in charge of the simulations of the experiment, “so this is strong evidence that the muon is sensitive to something that is not in our best theory.”

The predecessor experiment of Muon g-2, carried out in the Brookhaven National Laboratory (also from the US Department of Energy) and which ended in 2001, already offered indications that the behavior of the muon was not in accordance with the standard model. Now the new measurement at Fermilab agrees strongly with the value found at Brookhaven and diverges from theory with the most accurate measurement made to date.

Specifically, the accepted theoretical value for the g factor of the muon is 2.00233183620 and its anomalous magnetic moment 0.00116591810. However, the new experimental results announced this week by the Muon g-2 collaboration are a g factor of 2.00233184122 and an anomalous magnetic moment of 0.00116592061.

One chance in 40,000 that it is chance

The difference seems very small, but the combined data from Fermilab and Brookhaven show a discrepancy with the theory that presents a standard deviation or significance of 4.2 sigma, a little less than the 5 sigmas that scientists require to claim a true discovery, but still compelling evidence for new physics. The probability that the results are a statistical fluctuation is approximately 1 in 40,000.

The first results from Fermilab’s Muon g-2 experiment confirm those conducted two decades ago at Brookhaven National Laboratory. Both show strong evidence that muons deviate from the prediction of the standard model of particle physics. / Ryan Postel, Fermilab / Muon g-2 collaboration

The Fermilab experiment reuses the main component of the Brookhaven experiment, a superconducting magnetic storage ring 15 meters in diameter. In the Muon g-2 facilities, a beam of muons is sent to this ring, where they circulate thousands of times at almost the speed of light, and the detectors that cover it allow the precession speed of the muons to be determined.

The discrepancy with the theory has a standard deviation of 4.2 sigmas, slightly less than the 5 sigmas that scientists require to confirm a true discovery, but still compelling evidence for new physics.

In its first year of operation, in 2018, the Fermilab experiment collected more data than all previous experiments on muon g factor combined. With more than 200 scientists from 35 institutions in seven countries, the Muon g-2 collaboration has now finished analyzing the movement of more than 8 billion muons from their first run or execution phase.

“After the 20 years that have passed since the Brookhaven experiment ended, it is very gratifying to finally solve this mystery,” emphasizes the Fermilab scientist. Chris polly, who was a student in the old experiment and is now a co-spokesperson for the current one.

Analysis of the data from the second and third run of de Muon g-2 is currently underway, the fourth phase is also underway and a fifth is already planned. Combining the results of the five runs will provide scientists with an even more accurate measurement of muon wobble, revealing with greater certainty whether new physics is indeed hiding within the quantum foam.

“So far we have analyzed less than 6% of the data that the experiment will end up collecting,” says Polly, who like her colleagues is looking forward to what may come: “Although these first results tell us that there is an intriguing difference with the standard model, we’re going to learn a lot more in the next two years. “

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