Unstable helium adds a limit on the ongoing saga of the proton’s size

Enlarge / The small particle accelerator in Switzerland where, surrounded by farms, the work took place. (credit: Paul Scherrer Institut)

Physicists, who dedicate their lives to studying the topic, don't actually seem to like physics very much since they're always hoping it's broken. But we'll have to forgive them; finding out that a bit of theory can't possibly explain experimental results is a sign that we probably need a new theory, which is something that would excite any physicist.

In recent years, one of the things that has looked the most broken is a seemingly simple measurement: the charge radius of the proton, which is a measure of its physical size. Measurements made with hydrogen atoms, which have a single electron orbiting a proton, gave us one answer. Measurements in which the electron was replaced by a heavier particle, called a muon, gave us a different answer-and the two results were incompatible. A lot of effort has gone into eliminating this discrepancy, and it has gotten smaller, but it hasn't gone away.

That has theorists salivating. The Standard Model has no space for these kind of differences between electrons and muons, so could this be a sign that the Standard Model is wrong? The team behind some of the earlier measurements is now back with a new one, this one tracking the behavior of a muon orbiting a helium nucleus. The results are consistent with other measurements of helium's charge radius, suggesting there's nothing funny about the muon. So the Standard Model can breathe a sigh of relief.

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