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RandomFactor writes:

New measurements of Proton radius have shown that 2010 measurements, which showed an unknown interaction between protons and muons causing a puzzling reduction in proton radius, were in error.

Almost ten years ago German physicists reported a reduction in the radius of Protons when electrons were replaced with Muons in Hydrogen atoms. This lead to hopes for new physics (the best kind) but these hopes have now been dashed.

If the discrepancy was real, meaning protons really shrink in the presence of muons, this would imply unknown physical interactions between protons and muons—a fundamental discovery. Hundreds of papers speculating about the possibility have been written in the years since.

But hopes that the "proton radius puzzle" would upend particle physics and reveal new laws of nature have now been dashed by a new measurement reported on September 6 in Science.

The new measurements by Distinguished Research Professor Eric Hessels of York University in Toronto and his team "suggest that the proton does not change size depending on context; rather, the old measurements using electronic hydrogen were wrong."

According to Hessels, the result, which "points to the most mundane explanation" is bittersweet.

See also: https://arstechnica.com/science/2019/09/physics-not-broken-after-all-were-close-to-resolving-proton-radius-puzzle/.

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