Scientists working at Fermilab in Batavia, Illinois have made some of the most important discoveries in physics over the years, including the existence of the top quark and characterizing the neutrino. Now, the team working on Fermilab’s Muon g−2 experiment has reported a tantalizing hint of a new type of physics, according to the BBC. If confirmed, this would become the fifth known fundamental force in the universe.
Our current understanding of particle physics is called the Standard Model, which we know is an incomplete picture of the universe. Concepts like the Higgs boson and dark energy don’t fully integrate with the Standard Model, and the Muon g−2 might eventually help us understand why. The key to that breakthrough could be the behavior of the muon, a subatomic particle similar to an electron. The muon has a negative charge, but it’s much more massive. So, it spins like a magnet, which is what points to a possible new branch of physics.
The roots of the Muon g−2 experiment go back to work done at CERN in the late 1950s. However, the instruments available at the time were too imprecise to accurately measure the “g-factor” of the muon, which describes its rate of gyration. The Standard Model predicts that muons wobble in a certain way, but the 14-meter magnetic accelerator at the heart of Muon g−2 shows that muons have a different g-factor. That might not sound significant, but even a tiny “anomalous magnetic dipole moment,” as scientists call it, could indicate something mysterious has affected the particles.
The 600-ton g-2 magnet before installation.
We currently know of four fundamental forces: gravity, electromagnetism, the strong force (nuclear cohesion), and the weak force (radioactive decay). Whatever is causing muons to misbehave in Muon g−2 could be a fifth force, but we don’t know what it is. Even if the team can confirm the result, we won’t necessarily know what this new force of nature does aside from perturbing muons. That part will take much more work. Theoretical physicists have speculated that the new force could be associated with an undiscovered subatomic particle like the Z-prime boson or leptoquark.
The current focus is on improving the precision of the experiment. The new result was reported with a statistical confidence of 4.1 sigma, which works out to a 1 in 40,000 chance that the results are just statistical noise. Traditionally, scientists want to see a 5 sigma confidence (about 1 in 3.5 million) before calling something confirmed. This is something physicists are going to be talking about a lot in the coming months.
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