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February 20, 20204:00 pm – 5:00 pm (CDT)

The Lamb Shift and the Proton Radius: How Nature Doesn’t Care About Our Fond Hopes


Amar Vutha (University of Toronto)


Roland Allen



Mitchell Institute for Fundamental Physics & Astronomy

College Station, Texas 77843

Event Details

For the last ten years, there was a persistent problem regarding the size of the proton. Strangely, the proton’s charge radius measured using electrons seemed to be different than the radius measured using muons – this was the “proton radius puzzle” [1]. To resolve this puzzle, we began an experiment in 2011 to carefully measure the proton radius using the Lamb shift in hydrogen, and concluded it recently [2]. I will tell the story of this experiment, describing the initial motivations, the new tools and techniques that we had to invent for it, and some of the joys (and pains) of such a precision measurement. The proton radius puzzle and its eventual resolution might also offer some interesting insights into the scientific method.

1. D Castelvecchi, Nature 575, 269 (2019) 2. N Bezginov et al. Science 365, 1007 (2019)

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