Muons: the fundamental particles that reveal a new physics

Early results from the Muon g-2 experiment at Fermilab have reinforced the evidence for new physics. The centerpiece of the experiment is a 50-foot-diameter superconducting magnetic storage ring, which sits in its detector room amid electronic racks, the muon beamline, and other equipment. This impressive experiment operates at negative 450 degrees Fahrenheit and studies the oscillation of muons as they travel through the magnetic field. (Photo: Fermilab)

The first results of the Muon g-2 experiment at the Fermi National Accelerator Laboratory (Fermilab) of the US Department of Energy dispelled a doubt that scientists had had for decades.

The fundamental particles known as muons are not shared according to the theory of the Standard Model of Particle Physics. What does it mean? that act as a window to the subatomic world and could interact with undiscovered particles or forces.

“It is an extraordinary day, long awaited, not only by us but by the entire international physical community,” he said. Graziano Venanzoni, co-spokesperson for the Muon g-2 experiment and physicist at the Italian National Institute of Nuclear Physics.

“Much of the credit goes to our young researchers who, with their talent, ideas and enthusiasm, have allowed us to achieve this incredible result.”

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What are muons?

A muon is about 200 times more massive than its cousin, the electron. Muons are produced naturally when cosmic rays strike the Earth’s atmosphere and Fermilab’s particle accelerators can produce them in large quantities.

Like electrons, muons act as if they have a small internal magnet.

In a strong magnetic field, the direction of the muon magnet wobbles, much like that of axis of a spinning top a gyroscope.

The force of the internal magnet determines the speed of the muon in an external magnetic field and is described by a number that physicists call the g-factor. This number can be calculated with ultra-high precision.

As the muons circulate in 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 accounted for in the Standard Model, that would further alter the muon’s g-factor.

“This quantity that we measure reflects the interactions of the muon with everything else in the universe. But, When theorists calculate the same quantity, using all known forces and particles in the Standard Model, we do not get the same answer“Said Renee Fatemi, a physicist at the University of Kentucky and simulation manager.

“This is strong evidence that the muon is sensitive to something that is not in our best theory,” he said.

The previous experiment at DOE’s Brookhaven National Laboratory, which was completed in 2001, offered indications that the behavior of the muon did not agree with the Standard Model. The new measurement from the Muon g-2 experiment at Fermilab is in strong agreement with the value found at Brookhaven and differs from theory with the most accurate measurement to date.

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Evidence of new physics

The combined results from Fermilab and Brookhaven show a difference from theory with a significance of 4.2 sigma, slightly below the 5 sigma (or standard deviations) that scientists require to claim a discovery, but still compelling evidence for new physics.

The probability that the results are a statistical fluctuation is approximately 1 in 40,000.

The Fermilab experiment reuses the main component of the Brookhaven experiment, a magnetic storage ring 50-foot diameter superconductor.

In 2013, It was transported 3,200 miles by land and sea from Long Island to the Chicago suburbs, where scientists were able to take advantage of Fermilab’s particle accelerator and produce the most intense muon beam in the United States.

For the next four years, the researchers set up the experiment; tuned and calibrated an incredibly uniform magnetic field; developed new techniques, instrumentation, and simulations; and thoroughly tested the entire system.

The Muon g-2 experiment sends a beam of muons into the storage ring, where they circulate thousands of times at almost the speed of light. Detectors lining the ring allow scientists to determine how fast the muons are precessing.

In its first year of operation, in 2018, the Fermilab experiment collected more data than all previous muon g-factor experiments combined.

With more than 200 scientists from 35 institutions in seven countries, the Muon g-2 collaboration has finished analyzing the movement of more than 8 billion muons from that first run.

“After the 20 years that have passed since the Brookhaven experiment ended, it is very gratifying to finally be solving this mystery,” said the Fermilab scientist, Chris polly, another co-spokesperson for the current experiment and was a leading graduate student in the Brookhaven Experiment.

Data analysis for the second and third runs of the experiment is underway, the fourth run is in progress, and a fifth run is planned.

Combining the results of the five runs will give scientists an even more accurate measure of the muon oscillation, revealing with greater certainty whether the new physics is lurking within the quantum foam.

“So far we have analyzed less than 6% of the data that the experiment will eventually collect. Although these early results tell us that there is an intriguing difference from the Standard Model, we will learn much more in the years to come, ”said Polly.