Why did a US$3-million-winning result leave one physicist feeling like Peanut’s character Charlie Brown, scuppered from kicking the football at the last second by his friend Lucy?
I recently wrote a story for Nature about this year’s Breakthrough Prize winners, including the several hundred recipients of the Fundamental Physics prize awarded to those who worked on the Muon g-2 experiment series, that took place first at CERN, near Geneva, Switzerland, then at the Brookhaven National Laboratory in Long Island, New York, and finally at Fermilab, in Batavia, Illinois. The experiments endeavored to pin down the value of the muon’s magnetic moment, its internal quantum property that makes it behave like a little bar magnet. When placed in a magnetic field it precesses or wobbles, like the axis of a child’s spinning top. There had long been intriguing hints that the amount of wobble deviated from the Standard Model’s predictions, potentially pointing to new physics. So particle physicists had been keen to keep upping the precision of experiments to check.
I had a lovely chat with David Hertzog of the University of Washington in Seattle, who worked on the experiment at Brookhaven and then later led follow-up measurements at Fermilab. He noted that when Fermilab reported the final results in 2025—to a precision of 127 parts in a billion—most news outlets reported that the experiment confirmed the Standard Model’s predictions, end of story. He was slightly frustrated because things are not quite that simple.
The wobble is quantified by the particle’s g-factor, which for a long time was assumed to be exactly 2, but early experiments showed a slight deviation. This shift was explained by a peculiar quantum effect that causes virtual particles to briefly pop into existence, nudging the muon, before disappearing. Brookhaven’s experiments made headlines in 2001 and 2006, when Hertzog and colleagues found additional discrepancies with theory—possibly pointing to the existence of undiscovered particles or forces. “It was tantalising,” says Hertzog, but shy of the five sigma deviation needed to claim a definitive mismatch.
The team proposed follow-up tests at Brookhaven, but by then the lab had switched focus. So, to boost precision, Hertzog and others moved the experiment to Fermilab, famously transporting its 50-foot wide magnet by truck and barge down the east coast, around Florida and up the Mississippi River. The move paid off, with Fermilab poised to report preliminary results suggesting a 5.8 sigma deviation from the Standard Model, in 2020. But before their moment of glory, the theoretical predictions were drastically updated, thanks to a new kind of “lattice” calculation that used computer simulations to re-derive the influence of a particular type of virtual particle. If you want more details on those lattice calculations, listen to me chatting with quantum physicist Ian Durham about them, a couple of years ago on the FQxI podcast.
The new lattice calculations seemingly brought theory and experiment back together. “It was my Charlie Brown moment,” Hertzog joked to me.
Yet, the football has not been entirely swiped away by Lucy (or the theoretical physicists carrying out the lattice calculations). Despite the fact that follow-up lattice calculations continue to agree with its predictions, the Standard Model should not rest easy, Hertzog cautioned. He noted that there’s no clear reason why the updated lattice calculations give profoundly different predictions to the older-style calculations, which are based on real data from multiple particle accelerators around the globe. Their disagreement may itself point to new physics.“We’re in limbo,” says Hertzog. “We won’t know for a few more years where we will land.”
Read more about this award and this year’s other Breakthrough prize winners in my story over at Nature, including Nobel Laureate David Gross, who was awarded a special prize in fundamental physics, three sets of life scientists whose work has led to advances towards gene therapies for inherited retinal diseases, ALS and frontotemporal dementia and sickle-cell disease and beta-thalassemia and mathematician Frank Merle for his work on non-linear equations, with applications to quantum physics and fluid dynamics.
Lead image credit: Brookhaven National Laboratory. The Muon g-2 ring is lifted from the specially adapted truck at Smith Point Marina on Long Island on June 24, 2013. The entire apparatus weighs about 50 tons.