Across more than six decades, scientists and engineers from three “muon g-2” collaborations, representing dozens of institutions, have pushed experimental precision ever higher in pursuit of a single, very significant number: the anomalous magnetic moment of the muon. The muon is a heavy, unstable cousin of the electron, and like the electron it can behave like a tiny magnet. The physicists are looking to capture how the muon's magnetic strength is subtly affected by the “foam” of virtual particles constantly popping in and out of empty space around it. Measuring the muon’s magnetism and comparing it to theoretical predictions allows physicists to test whether any unknown particles or forces are hidden in this foam. In other words, to probe for new physics beyond the Standard Model, our most successful theory of particles and forces.

The CERN collaboration’s pioneering storage ring experiments of the 1960s and 1970s first measured the anomalous magnetic moment with meaningful precision. Then in the 1990s, Brookhaven National Laboratory’s reimagining of the experiment achieved a major improvement in precision. And after the audacious transportation of Brookhaven's 50-ton, 15-meter-diameter storage ring 3,200 miles by road and barge to Fermilab in 2013, the experiment was systematically refined to achieve a final precision of 127 parts per billion – a mind-boggling 30,000 times more precise than the first g-2 experiment in 1965. The results had shown a tantalizing discrepancy with the value predicted by theory; and in 2023, Fermilab’s new results pushed that discrepancy close to the threshold considered evidence for new physics. Since then, the final, even more precise results, compared to newly evolved theoretical calculations narrowed the gap, but considerable uncertainty remains for the moment. Whatever the final verdict, this experiment represents a remarkable theoretical, experimental and technological endeavor, achieving extraordinary precision in the quest for fundamental understanding.