Particle physicists are faced with a growing list of “anomalies”—experimental results that conflict with the standard model but fail to overturn it for lack of sufficient evidence.
To go beyond the realm of established theory, particle physicists are ready to move heaven and earth—and even giant magnets. In 2013, researchers packed up a circular magnet the width of a basketball court and sent it on a 3200-mile trip from New York to Illinois. Over the course of 35 days, the 15-ton magnet sailed down the East Coast, rounded the tip of Florida, floated up the Mississippi, and rode on the back of a truck to Fermilab, where it now serves as the central element of the revamped Muon g-2 experiment. Particle physicists went through this colossal effort to investigate a 3-parts-per-billion disagreement between theory and experiment over the value of the muon’s magnetic moment.
Although it may seem small, this discrepancy is one of the longest-standing anomalies in particle physics. Here, “anomaly” means a statistically significant experimental divergence from theoretical prediction. The theory in this case is the standard model of particle physics—a schema for all the known particles and forces besides gravity. The muon anomaly is not alone in contesting the standard model: other anomalies concern bottom quarks, neutrinos, and kaons. In recent years, these anomalies have taken on a new level of importance as possible routes to “new physics,” an umbrella term for phenomena unexplained by the standard model. There are more conspicuous paths to new physics, such as discovering a dark matter particle or unifying gravity with quantum physics. But these big problems have remained stubbornly out of reach, so many particle physicists are looking for inspiration from smaller problems.