Inside a sealed envelope, a single number held the potential to reshape our understanding of particle physics. This value, a meticulously concealed time measurement from the Muon g-2 experiment at Fermilab, was the key to a calculation that could either resolve a long-standing mystery or deepen it further. Recently, 170 scientists gathered virtually to witness the unveiling of this crucial piece of information.
The atmosphere was electric with anticipation. “The moment the number appeared on the screen, a wave of relief, excitement, pride, and joy washed over us,” recounts Sudeshna Ganguly, associate scientist at Fermilab. “We had to unmute ourselves just to cheer.”
The Muon g-2 Collaboration has now publicly released their highly anticipated findings, detailing the behavior of muons—heavier cousins of electrons—within a magnetic field. This new measurement aligns remarkably well with previous results from Brookhaven National Laboratory, creating a significant discrepancy with predictions from the Standard Model, the cornerstone theory of particle physics. The combined experimental results now deviate from the Standard Model’s calculations by a compelling 4.2 standard deviations.
While not yet reaching the “five sigma” threshold (a one in 3.5 million chance of the observed discrepancy occurring randomly), this 4.2 sigma deviation is a tantalizing hint that something might be amiss in our current understanding of fundamental particles. The “g-2” value represents a minute correction to the muon’s magnetic moment, a property describing how the particle wobbles in a magnetic field.
The quest to understand g-2 dates back to 1928 with Paul Dirac’s calculation of the electron’s magnetic moment. Subsequent refinements by physicists like Julian Schwinger led to the concept of g-2. Experiments to measure muon g-2 followed at Columbia’s Nevis Laboratories, CERN, and then Brookhaven National Lab, where initial hints of a discrepancy emerged in 2004.
To further investigate this intriguing anomaly, Brookhaven’s massive 50-foot g-2 ring was transported to Fermilab in 2013, embarking on a remarkable journey by barge and truck. The experiment resumed at Fermilab in 2017, utilizing the lab’s powerful accelerators to create a beam of antimuons (the antiparticle of the muon).
The experiment involves circulating these antimuons within the electromagnetic ring at near-light speed. As they decay, they emit positrons, whose detection provides insights into the antimuons’ behavior in the magnetic field, and thus their g-2 value. While antimuons are easier to produce, the g-2 value remains the same for both muons and antimuons.
The source of the discrepancy remains a mystery. “Perhaps it’s a particle that’s difficult to create,” speculates Fermilab’s deputy director of research, Joe Lykken. “If so, it might be detectable through cosmic observations or hidden within existing data.” This discrepancy could potentially connect to other unresolved puzzles in physics, such as dark matter, the Hubble tension, or recent findings from CERN’s LHCb experiment.
The new results have generated excitement within the physics community. “Two independent experiments showing over three standard deviations from the predicted value strongly suggests the experimental measurement is robust,” remarks Freya Blekman, a particle physicist at Vrije Universiteit Brussel. “The onus now falls on theoretical physicists to re-evaluate their calculations.”
While a recent theoretical paper offered a potential explanation for the discrepancy, it also introduced new inconsistencies. Chris Polly, co-spokesperson for the Muon g-2 experiment, emphasizes the significance of their precise measurements, stating that any viable physics theory must account for them.
The COVID-19 pandemic presented significant challenges, limiting on-site personnel and requiring remote operation of equipment like the magnetic field-mapping trolley. Despite these obstacles, the geographically dispersed team adapted, utilizing their existing remote collaboration infrastructure to analyze the data.
The Muon g-2 Collaboration is celebrating their achievement but acknowledges the work ahead. With only 6% of the data analyzed, future runs and refinements promise even more precise measurements. “It’s exhilarating to be a part of this,” says Ganguly. The pursuit of a more precise muon g-2 value continues, potentially unlocking new insights into the fundamental building blocks of the universe.