An inexplicable result of the muon experiment opens the door to new physics

An inexplicable result of the muon experiment opens the door to new physics

The g-2 experiment in Fermilab

The g-2 experiment in Fermilab
picture: Fermilab

inside Closed lockerOne of the envelopes contained a number that was about to destabilize material society, regardless of its content. The value, a clock scale intentionally hidden to keep physicists’ data analysis unbiased, can be used in a calculation that could put an end to one of the main mysteries of particle physics, or make it more enigmatic. On a recent video call, 170 scientists gathered to watch the envelope opening.

“When we saw the number on the screen, we felt a lot of relief, excitement, pride and joy,” said Sudhna Ganguly, associate scientist at Fermilab. “We had to be silent to be able to shout.”

Today, the global team of scientists from Fermilab’s Muon g-2 experiment published one of the most anticipated measurements in particle physics, describing the behavior of the electron’s heavier cousin, the muon, in the magnetic field. If other studies confirm their results, the researchers will face an inexplicable contradiction between their experimental work and the fundamental theory of particle physics, the Standard Model.

Physicists are making highly-accurate measurements of the properties of particles as a way to probe the foundations of the universe. Magnetic moment is one of those properties that describes how particles move, or oscillate like a spinning top, in the presence of a magnetic field. Previous measurements at Brookhaven National Laboratory in New York indicated that measurements of the muon’s magnetic moment may not match predictions of the Standard Model. Today, the Muon g-2 Collaboration announced that they have measured a value that closely matches Brookhaven’s, in disagreement with the Standard Model. Taken together, the experimental measurements are inconsistent with calculations of the Standard Model of 4.2 standard deviations.

This discrepancy is not strong enough to satisfy the statistical parameter of five standard deviations (called “five sigma”) that particle physicists use to indicate that they have already discovered something. Five sigma means that the probabilities that particles following the rules of the Standard Model will produce the measured value after the experiment period is about 1 in 3.5 million. In other words, if experimenters arrivedAt the five sigma confidence level, they can be fairly certain that the Standard Model is missing a chunk to explain the value of g-2.

The g-2 measurement dates back to 1928, when physicist Paul Dirac calculated that the magnetic moment of the electron, g, was exactly 2. However, deeper searches for the property by physicists such as Julian Schwinger led to corrections. g and 2. Experiments followed for measuring g-2 muon, first in Columbia Nevis Laboratories And the CERN, And then at Brookhaven National Laboratory, whose collaboration ended taking action in 2001 and released its final results, evidence of inconsistency, in 2004.

A decade later, scientists coordinated the National Laboratory to continue efforts and transported the fragile 15-meter ring of Brookhaven’s g-2 experiment from Long Island, New York, to Fermilab, Illinois, first on a battleship. Across the Atlantic and down the Mississippi, Then in the back of the truck. Scientists have been conducting the experiment on Fermilab since 2017.

The experiment begins with firing protons from fermilab accelerators at a fixed target, producing more protons and an antimonial particle, which is called an antagonist (essentially, a muon that behaves like its mirror image with the opposite charge), and another particle called a pion, which decays into antimony. This muon beam rotates around the electromagnetic ring to experiment at nearly the speed of light. Meanwhile, the antimony begins to decompose outside the beam as anti-electrons (also known as positrons) to reach the detectors. Measuring these positrons allows scientists to determine how the antimony behaves in the magnetic field and thus the g-2 value of the antimony. Antimuons are easier to produce compared to muons, but the g-2 value will be the same for both.

The reason for the discrepancy is not clear. “It might be difficult to produce a particle,” Joe Leakin, theoretical physicist and deputy research director at Fermilab, told Gizmodo in a video call. If so, “it should appear somewhere else, like cosmic observations. Or it might actually appear in our data and we need to uncover it.” There are many unresolved cosmic mysteries and contradictions (dark matter, and Hubble dynasty Or recent results LHCb experiment CERN, but not limited to) that theorists may attempt to relate to the g-2 muon paradox.

Researchers in the particle physics community are excited about the news. “The fact that two independent experiments see more than three standard deviations from the stated prediction means, to me, that the experimental measurement is powerful,” said Freya Blakeman, professor of elementary particle physics at the Free University of Brussels. , In a direct message from Twitter. “But the ball is really now in the court of theoretical physicists calculating the value it is compared to.”

Theorists have spent the past two decades trying to understand this paradox and what it could cause. Some question whether the discrepancy is real. Controversial article He calculated the value of muon g-2 in a different way that seems to explain the discrepancy. But even that account, if verified and accepted by the broader theoretical community, would present another set of discrepancies, said Chris Polley, Muon g-2 spokesperson. And Polly explained that the Fermilab experiment would be valuable whether the discrepancy was as real as it were not; Any physical theory that attempts to solve the mysteries of the universe must also match the team’s precise measurements.

Of course, the ongoing pandemic is also a challenge for scientists. Precautions around covid-19 have limited the number of researchers who can sit inside the experiment’s control room. “When you coordinate an experiment during normal business hours, you are always in the control room and in the experiment room telling people what to do,” Ganguly said. But in the past year, the number of people who can enter the test room and fix things when they break down has decreased. “Doing that with Zoom was challenging. But in the end, we had to make it work.”

The team operated everything they could remotely, such as the vehicle running inside the experiment, to track its magnetic field. Social distancing had little effect on data analysis. The team represents scientists from countries around the world who are already accustomed to doing much of their work remotely, Polley said.

Today, the Muon g-2 Collaboration researchers are celebrating, but they have a lot more work to do. Through further analyzes, improvements and trial runs, the team plans to further reduce the experimental error to more accurately calculate the g-2 muon value.

“So far, we have analyzed only 6% of the data, and when we collect the results of all the runs, we will get a better scale,” Ganguly said. “It’s very exciting to be a part of this.”

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Zoe Marsh

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