Physicists Finally Nail the Proton’s Size, and Hope Dies

Wired | 9/15/2019 | Staff
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In 2010, physicists in Germany reported that they had made an exceptionally precise measurement of the size of the proton, the positively charged building block of atomic nuclei. The result was very puzzling.

Randolf Pohl of the Max Planck Institute of Quantum Optics and collaborators had measured the proton using special hydrogen atoms in which the electron that normally orbits the proton was replaced by a muon, a particle that’s identical to the electron but 207 times heavier. Pohl’s team found the muon-orbited protons to be 0.84 femtometers in radius—4 percent smaller than those in regular hydrogen, according to the average of more than two dozen earlier measurements.

Story - Permission - Quanta - Magazine - Publication

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.

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.

Proton - Radius - Puzzle - Particle - Physics

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.

After Pohl’s muonic hydrogen result nine years ago, a team of physicists led by Eric Hessels of York University in Toronto set out to remeasure the proton in regular, “electronic” hydrogen. Finally, the results are in: Hessels and company have pegged the proton’s radius at 0.833 femtometers, give or take 0.01, a measurement exactly consistent with Pohl’s value. Both measurements are more precise than earlier attempts, and they suggest that the proton does not change size depending on context; rather, the old measurements using electronic hydrogen were...
(Excerpt) Read more at: Wired
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