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Proton radius from muonic hydrogen
Pohl, Randolf Laser Spectroscopy Division, Max Planck Institute of Quantum Optics, Garching, Germany.
- Proton charge radius
- Elastic electron-proton scattering
- Hydrogen spectroscopy
- Muonic hydrogen spectroscopy
- Proton radius puzzle
- Physics beyond the Standard Model
- Related Primary Literature
- Additional Reading
The recent measurement of the Lamb shift (2S-2P energy difference) in muonic hydrogen (μp) resulted in a new determination of the root-mean-square (rms) charge radius of the proton, rp = 0.8418 ± 0.0007 femtometer (1 fm = 10−15 m), which is 10 times more accurate than any previous determination from elastic electron-proton (e-p) scattering or precision spectroscopy of energy levels in the hydrogen atom (H), but differs by seven standard deviations from these measurements (Fig. 1). This discrepancy is now known as the “proton radius puzzle.” This article reviews the traditional methods for the determination of rp, describes the muonic hydrogen experiment, and reviews the attempts to resolve the puzzle, including suggestions for physics beyond the standard model.
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