Long-awaited results from America's Fermi National Accelerator Laboratory, its particle physics and accelerator laboratory, better known as Fermilab, have strengthened the evidence for brand new physics by showing fundamental particles not behaving in a way predicted by our best theory, the Standard Model of Particle Physics predicts.
The Muon g-2 experiment has announced its first results and independently confirmed previous observations. The collaboration found that the measured value of the muon magnetic moment is different from what the Standard Model predicts. The findings have reached the threshold of 4.2 sigma uncertainty, close to the "gold standard" level of certainty in scientific evidence of 5 sigmas, but not quite there yet. It still means that there's a 3-in-100,000 chance that this is a fluke.
While yet to be absolutely confirmed, the finding is the strongest evidence yet that not only is there unknown physics out there but that we know a very good place for where to start looking for it, which is very exciting.
“Today is an extraordinary day, long-awaited not only by us but by the whole international physics community,” said Graziano Venanzoni, co-spokesperson of the Muon g-2 experiment and physicist at the Italian National Institute for Nuclear Physics. “A large amount of credit goes to our young researchers who, with their talent, ideas and enthusiasm, have allowed us to achieve this incredible result.”
The Muon g-2 experiment has been investigating the anomalous magnetic dipole moment of the muon, a particle similar to the electron but 207 times more massive. The experiment measures how strong the internal magnetism of the muon really is, something we have a clear prediction of from theory. But previous measurements offered hints of a value very different from what was expected.
There was some room for uncertainties so one possibility was that the measurement was just a statistical fluke. Alternatively, the experiment and/or the analysis had some unknown issues. The most exciting was the last explanation: the findings were due to forces or particles that have not been predicted in our theories.
To get a clearer understanding of what was going on, physicists had to literally move the experiment about 5,000 kilometers (3,200 miles) across America. The original results announced back in 2006 were from the Brookhaven National Laboratory in Long Island, New York. But the researchers needed a bigger muon accelerator. Fermilab in Chicago had one, so they decided to move the experiment there as it would be cheaper than building a new accelerator.
The huge Muon g-2 electromagnet went down to Florida and then via the Tennessee-Tombigbee riverways and Illinois River until it reached Chicago.
The experiment at Fermilab started in 2017 and lasted for about three years. In total, it analyzed 8 billion muons, 21 times more data than the original experiment, leading to a measurement 10,000 times more precise. And this is only from the first run of the experiment. Data from runs two and three are currently being analyzed, while the fourth run is currently underway.
“After the 20 years that have passed since the Brookhaven experiment ended, it is so gratifying to finally be resolving this mystery,” said Fermilab scientist Chris Polly, who is a co-spokesperson for the current experiment and was a lead graduate student on the Brookhaven experiment. “So far we have analyzed less than 6 percent of the data that the experiment will eventually collect. Although these first results are telling us that there is an intriguing difference with the Standard Model, we will learn much more in the next couple of years.”