GoKawiilThat makes the precise size of the excess wobble, the muon’s “g–2,” invaluable as a window into the quantum world. “The measurement of muon g–2 is a proxy for saying how many particles exist in the universe,” said Alex Keshavarzi, a senior research fellow at University College London.
So when an experiment at Brookhaven National Laboratory on Long Island measured the muon’s g-factor in 2001, physicists were thrilled that it came out larger than expected. To some, it hinted that new particles — perhaps even particles that could account for dark matter — were at work.
Physicists set out to check the result with an even more precise measurement. In 2013, Brookhaven’s 50-foot-wide magnetic ring was moved via an elaborate series of barges and trucks to Fermi National Accelerator Laboratory (Fermilab) in Illinois, where an upgraded version of the experiment would take even more data.
To prepare for that new experiment, physicists also made a huge effort to understand the theoretical prediction that disagreed with the data. Their challenge was to understand the muon’s chains of emission and reabsorption in extreme detail. In particular, how much do the particles associated with each of nature’s four fundamental forces participate in these chains?
Alex Keshavarzi, a physicist at University College London, helped refine a way to infer how the muon should wobble from certain collider experiments. Courtesy of Alex Keshavarzi
The calculation is straightforward for three of nature’s four forces. Gravity is so weak that physicists can ignore it outright. And both the electromagnetic force and the weak nuclear force can be deduced using a standard technique.
The strong force, however, is not so easy to deal with. That force tightly binds particles known as quarks into composite particles such as protons and neutrons. Standard theoretical techniques don’t work on the strong force. So physicists have to get creative.
In his doctoral thesis in 2018, Keshavarzi helped hone an alternative way of understanding the strong force, called the data-driven method. In this method, physicists don’t try to predict how often muons will emit and absorb groups of quarks. They go out and measure it.
The main way that happens is by colliding electrons and their antimatter partners, positrons. The matter and antimatter annihilate each other, creating other particles, including bundles of quarks. If lots of quarks appear, physicists know they have a tight quantum link to particles such as electrons and positrons. In short, the more quarks appear in electron-positron collisions, the more strongly they will affect the muon.
Using the data-driven method, physicists set out to calculate the expected size of the muon’s magnetic wobble. That prediction, which was released in June 2020, sharply differed from Fermilab’s precise experimental measurement, which came out in April 2021. The discrepancy was so strong that it nearly crossed the stringent threshold required for physicists to claim they had discovered new particles.
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