Muons don’t fit the standard model of particle physics

Fundamental particles called muons behave in ways that scientists’ best theory to date, the Standard Model of particle physics, does not predict, the researchers report.

The discovery comes from early results from the Muon g-2 experiment at the US Department of Energy’s Fermi National Accelerator Laboratory.

“This experience is a bit like a detective novel.”

This historic result confirms a discrepancy that has plagued researchers for decades.

The strong evidence that muons deviate from the Standard Model calculation could hint at some exciting new physics. The muons in this experiment act as a window into the subatomic world and could interact with as yet unknown particles or forces.

“This experiment is a bit like a detective novel,” says team member David Hertzog, a University of Washington physics professor and founding spokesperson for the experiment. “We analyzed data from the inaugural Muon g-2 test at Fermilab and discovered that the Standard Model alone cannot explain what we found. Something else, perhaps beyond the standard model, may be required. »

A muon is about 200 times more massive than its cousin the electron. They occur naturally when cosmic rays hit the Earth’s atmosphere. Fermilab’s particle accelerators can produce large numbers of them. Like electrons, muons act as if they have a small internal magnet. In a strong magnetic field, the direction of the muon magnet precedes, or “wobbles,” much like the axis of a spinning top. The strength of the internal magnet determines the muon’s precession rate in an external magnetic field and is described by a number known as the g-factor. This number can be calculated with ultra-high precision.

As muons flow 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 muon precession to accelerate or slightly slow down. The standard model predicts with great accuracy what the value of this “abnormal magnetic moment” should be. But if the quantum foam contains additional forces or particles not accounted for by the Standard Model, it would further alter the g-factor of the muon.

Hertzog, then at the University of Illinois, was a lead scientist in the previous experiment at Brookhaven National Laboratory. This attempt ended in 2001 and offered clues that the behavior of the muon did not conform to the Standard Model. The new measurement from the Muon g-2 experiment at Fermilab strongly agrees with the value found at Brookhaven and deviates from theory with the most accurate measurement to date.

The accepted theoretical values ​​for the muon are:

  • g-factor: 2.00233183620(86)
  • abnormal magnetic moment: 0.00116591810(43)

The new experimental global mean results announced today by the Muon g-2 collaboration are:

  • g-factor: 2.00233184122(82)
  • abnormal magnetic moment: 0.00116592061(41)

The combined results from Fermilab and Brookhaven show a difference with theoretical predictions at a significance of 4.2 sigma, a little short of the 5 sigma – or 5 standard deviations – that scientists prefer as a claim of discovery. But it’s still compelling evidence of new physics. The probability that the results are a statistical fluctuation is approximately 1 in 40,000.

“This result from the first run of the Fermilab Muon g-2 experiment is arguably the most anticipated result in particle physics in recent years,” says Martin Hoferichter, assistant professor at the University of Bern and member of the theoretical collaboration that predicts the value of the standard model. “After almost a decade, it’s great to see this massive effort finally come to fruition.”

The Fermilab experiment, which is underway, reuses the main component of the Brookhaven experiment, a superconducting magnetic storage ring 50 feet in diameter. In 2013, it was transported 3,200 miles by land and sea from Long Island to suburban Chicago, where scientists were able to take advantage of Fermilab’s particle accelerator and produce the states’ most intense muon beam. -United. Over the next four years, researchers mounted the experiment; tuned and calibrated an incredibly uniform magnetic field; developed new techniques, instruments 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 near the speed of light. Detectors lining the ring allow scientists to determine how fast muons “wobble”.

Many Fermilab sensors and detectors have been built at the University of Washington, such as instruments to measure the muon beam as it enters the storage ring and to detect the telltale particles that appear when the muons decay . Dozens of scientists – including professors, postdoctoral researchers, technicians, graduate students and undergraduates – worked to assemble these sensitive instruments, then set them up and monitor them at Fermilab.

“The outlook for the new result has triggered a coordinated theoretical effort to provide our experimental colleagues with a robust and consensus prediction of the Standard Model,” says Hoferichter. “Future runs will motivate further refinements, to enable a conclusive statement whether physics beyond the Standard Model lurks in the muon’s anomalous magnetic moment.”

In its first year of operation, 2018, the Fermilab experiment collected more data than all previous muon g-factor experiments combined. The Muon g-2 collaboration has now completed the analysis of the movement of more than 8 billion muons from this first period.

Analysis of data from the second and third cycles of the experiment is in progress; the fourth is in progress and a fifth is planned. Combining the results from all five tests will give scientists an even more precise measurement of the muon’s ‘wobble’, revealing with greater certainty whether new physics is lurking in the quantum foam.

“So far, we’ve analyzed less than 6% of the data the experiment will eventually collect,” says Fermilab scientist Chris Polly, who is a co-spokesperson for the current experiment and was a graduate student. of the University of Illinois under Hertzog during the Brookhaven Experiment. “While these early results tell us there is an intriguing difference to the standard model, we will learn much more over the next two years.”

“With these exciting results, our team, especially our students, are excited to push hard on analyzing the remaining data and taking future data to achieve our ultimate goal of accuracy,” says Peter Kammel, research professor of physics at the University of Washington.

An article on the research appears in Physical examination letters. Hertzog will present the findings at a University of Washington Physics Department Symposium on April 12.

The Muon g-2 experiment is an international collaboration between Fermilab in Illinois and more than 200 scientists from 35 institutions in seven countries.

Source: University of Washington

Comments are closed.