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Wednesday, June 19, 2024

Science Corner – Journey to the tiniest frontier


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 are spinning in the same way, which makes them into tiny magnets. Physicists have now made incredibly precise measurements of the strength of those magnets, disagreeing with 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 (NY) 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 tell in which direction their spin is pointing. The spin axis points away from the field direction and it traces a circle around the field – it precesses. From the precession frequency one can calculate the strength of those tiny magnets.

The Brookhaven result disagreed with 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 ten years ago the huge magnet ring was transported 3,200 miles to Fermilab – by ship down the Atlantic coast, up the Mississippi and by roads over three nights. Thousands came out to welcome it here. An improved version of the experiment was done, and the first results were announced in April 2021. They confirm the difference between measurement and the 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 is not truly isolated, it is surrounded by a “fuzz” of so-called virtual particles popping into temporary existence, borrowing energy briefly by Heisenberg’s uncertainty principle. Theorists learned how to calculate their effect very precisely, including all known fundamental particles that could affect it. For the electron, 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 9th 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 reason.

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Michael Albrow
Michael Albrow
Michael Albrow is a scientist emeritus at Fermilab, Batavia and a member of Naperville Sunrise Rotary. Born in England, Mike lived in Switzerland and Sweden before settling in the U.S. 25 years ago.