Physics Colloquium

Peter Mohr
National Institute of Standards and Technology

The Proton Radius Puzzle: An Unsolved Mystery

In July 2010, a paper in Nature reported a measurement made in Switzerland of the Lamb shift in muonic hydrogen, a negative muon bound to a proton, that may be considered a measurement of the proton radius. This value for the radius differs by seven standard deviations from the 2010 CODATA recommended value. The disagreement is a puzzle, and could conceivably turn out to be a fundamental shortcoming of quantum electrodynamics (QED). This would also be bad news for the fundamental constants, because many of the values rely on the assumption that the theory is correct.

Monday, 25 November, 2013


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