A muon's wobble tracks the particles tugging on it, but the count is disputed
A newer calculation matched the large wobble seen at Fermilab, yet an older method still predicts a smaller one.
Quanta Magazine · Physicists Solve a Big Quantum Mystery. Now, Old Results Don’t Add Up.The muon is a heavier relative of the electron. Set it circling in a magnetic field and it wobbles, tracing smaller circles inside its path, as a spinning bar magnet in a field would.
Physicists describe the size of those circles with a value known as the g-factor. Left entirely to itself, that number should equal two. But a muon is never alone, because quantum theory says empty space is not empty.
As the muon wobbles, it briefly gives off short-lived specks such as photons, packets of light too fleeting for detectors to catch. Those specks can spawn further specks, which can spawn still more, and the muon swiftly takes them all back. Physicists call this chain of particles that appear and vanish emission and reabsorption, and the only trace it leaves is a slightly larger wobble.
The extra wobble, called g-2, reflects every particle's small pull, so it can be used to measure them. That is the appeal for counting what exists.
The counting is uneven. Among the four basic forces, gravitation can be set aside as too feeble, while electromagnetism and the weak interaction behind some radioactivity yield to standard math. The strong force does not. It tightly binds specks called quarks into composite particles, for example protons and neutrons, and the usual equations fail there, so physicists must improvise.
One workaround is the data-driven method, refined in work including Alex Keshavarzi's 2018 doctoral thesis. Researchers do not calculate from scratch how often muons trade quark bundles back and forth. They collide electrons with positrons, their antimatter opposites, which destroy each other and turn into fresh debris. If many quarks emerge, it signals a close quantum connection to particles like electrons and positrons, which means those quarks will tug more strongly on the muon.
"The measurement of muon g-2 is a proxy for saying how many particles exist in the universe," said Alex Keshavarzi, a senior research fellow at University College London, where proxy means a stand-in. When Brookhaven National Laboratory found a larger than expected wobble in 2001, some researchers wondered if unseen particles were adding tugs.
A more precise test at Fermi National Accelerator Laboratory in Illinois confirmed the large wobble in April 2021, and a June 2020 data-driven forecast still came in smaller, almost enough to claim new particles. Yet that same day a group in Budapest, Marseille, and Wuppertal reported a different, grid-based calculation in which known particles fully explained the Fermilab result. Independent grids have since agreed, so many physicists think the original anomaly is gone, but the dispute has shifted to why the two calculations disagree and which collider count of quark bundles to trust.
Questions people ask
Why use the muon instead of the much more familiar electron?
Because it is much heavier than an electron, about 207 times as heavy, and sensitivity to brief heavy visitors rises with mass. That makes the muon tens of thousands of times more responsive than the electron to whatever might be appearing and vanishing nearby.
How big is the extra wobble?
Very small. The g-factor is just over 2, at about 2.00233, so the excess is about one part in a thousand of the full value. Physicists must measure and predict that thin excess to about one part in a million to tell if anything is missing.
What would dark matter have to do with a wobbling muon?
Cosmologists infer invisible material from the way galaxies move, and they estimate it outweighs ordinary matter by roughly five to one. If that material is made of undiscovered particles, as many researchers suspect, those particles should also join the brief crowd around a wobbling muon and add a tug.