Journey to the tiniest frontier

Photo portrait of a man with a gray and white beard and hair in a black collared shirt in front of an off-white background.

Mike Albrow

Fermilab hit the headlines in April, from the BBC to the New York Times — something to do with muons, tiny particles like electrons but about 200 times heavier, possibly opening a window to yet unknown particles or forces.

Muons have an electric charge like electrons, and both spin in the same way, which makes them into tiny magnets. Physicists have now made incredibly precise measurements of the strength of those magnets. But these measurements contradict equally precise calculations based on the current theory of particles and the forces between them. It seems there must be new particles or forces that we have not yet discovered.

A measurement was made at Brookhaven National Laboratory, in New York, 20 years ago. They stored a beam of muons in a 50-foot-diameter ring, keeping them in a circular path with a strong magnetic field. The muons decay into electrons as they circulate, and by detecting those electrons, scientists can measure the strength of the muons’ tiny magnets.

The Brookhaven result contradicted the theoretical prediction — very intriguing but not 100% convincing. It was crucial to repeat the experiment with more precision.

Fermilab took up the challenge and eight years ago the huge magnet ring was transported 3,200 miles to Fermilab — by ship down the Atlantic coast, up the Mississippi and Illinois rivers to Lemont, Illinois, and then by truck to Fermilab over three nights. Thousands came out to welcome it here. Then an improved version of the experiment was done using Fermilab’s particle accelerator complex, and the first results were announced in April. They confirm the tantalizing difference between measurement and theory, with more precision and with much more data to come.

The magnetic strength calculated by theory has a factor called “g,” only different from being exactly 2 because a muon does not exist in isolation. It is surrounded by a “quantum fuzz” of so-called “virtual particles” popping into temporary existence, borrowing energy briefly by Heisenberg’s uncertainty principle. Theorists learned how to calculate very precisely how this affects the strength of the tiny magnets, including all known fundamental particles that could contribute to the quantum fuzz. For the electron’s tiny magnet, theory and experiment agree to about 12 decimal places! For the muon, which is more affected by heavy particles, theory and experiment disagree in the ninth decimal place.

Now that the disagreement is very convincing, do some new particles await discovery? Time (and more data) will tell, but I hope that’s the case.

This is a version of an article that originally appeared in Positively Naperville. Mike Albrow is a Fermilab scientist emeritus. The author’s views are his own.