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Progress on the proton-radius puzzle

Atomic physicists and nuclear physicists have each made a refined measurement of the radius of the proton. Both values agree with a hotly debated result obtained by spectroscopy of an exotic form of hydrogen called muonic hydrogen.
  1. Jean-Philippe Karr
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  2. Dominique Marchand
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The proton, discovered 100 years ago1, is an essential building block of visible matter. The nucleus of a hydrogen atom consists of a single proton, making this atom a suitable platform for determining the proton’s intrinsic properties. One such property is the proton charge radius, which corresponds to the spatial extent of the distribution of the proton’s charge. In 2010, a highly accurate measurement of the proton radius was made using spectroscopy of muonic hydrogen — an exotic form of hydrogen in which the electron is replaced by a heavier version called a muon2. However, the value obtained was almost 4% smaller than the previously accepted one3. Bezginov et al.4, writing in Science, and Xiong et al.5, writing in Nature, report experiments that could represent a decisive step towards solving this proton-radius puzzle.

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Nature 575, 61-62 (2019)

doi: https://doi.org/10.1038/d41586-019-03364-z

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