![]() ![]() The Lamb shift for muonic hydrogen is thus much larger and easier to measure with a lower uncertainty. With a mass 207 times as large as an electron, a muon has a tighter orbit around the proton. But both the shift and the component due to the proton’s spatial extent are small.īack in 2010 Pohl and his colleagues measured the energy levels in muonic hydrogen, a proton orbited by a muon. By measuring the energy difference between a highly affected state and a relatively unaltered one-for example, the so-called Lamb shift between the 2s and 2p states-a researcher can determine the proton radius. The latter relies on the fact that lower-energy states don’t follow what we’d expect from Coulomb’s law, in part because the electron and proton can spatially overlap however, only states with the electron and proton close together are affected. The established proton charge radius was found through elastic electron–proton scattering experiments and hydrogen spectroscopy. Now two different approaches have yielded consistent smaller values for the proton radius and possibly resolved the mystery. The disagreement, known as the proton radius puzzle, opened the possibility of new physics to explain why and under what conditions the proton might behave differently. In the flurry of activity since, studies have supported both the old value and the new, smaller one for the charge radius. In 2010 Randolf Pohl of the Max Planck Institute of Quantum Optics in Garching, Germany, and his colleagues measured the proton radius as 0.84 femtometers, which is 5 standard deviations smaller than the accepted value of 0.88 fm. Although the proton doesn’t have definite boundaries, the charge radius is still well-defined in terms of the root mean square of the range of cross sections seen by other charged particles. 116, 053003 (2016).Nearly a decade ago, the value of the proton radius was unexpectedly thrown into doubt. Karr, and L. Hilico, “Theoretical hyperfine structure of the molecular hydrogen ion at the 1 ppm level,” Phys. Karr, “Fundamental transitions and ionization energies of the hydrogen molecular ions with few ppt uncertainty,” Phys. Karr, “Calculation of the relativistic Bethe logarithm in the two-center problem,” Phys. Sayan Patra, “Towards Doppler-free two-photon spectroscopy of trapped and cooled HD \(^\) and HD molecular ions,” Phys Rev. Korobov, and S. Schiller, “Rotational spectroscopy of cold and trapped molecular ions in the Lamb-Dicke regime,” Nat. Dicke, “The effect of collisions upon the Doppler width of spectral lines,” Phys. Sturm, “High-precision measurement of the proton’s atomic mass,” Phys. Newell, “CODATA recommended values of the fundamental physical constants: 2014,” Rev. Guéna, “New measurement of the \(1S\)– \(3S\) transition frequency of hydrogen: Contribution to the proton charge radius puzzle,” Phys. Udem, “The Rydberg constant and proton size from atomic hydrogen,” Science. ![]() “Improved measurement of the hydrogen \(1S\)– \(2S\) transition frequency,” Phys. Newell, “CODATA recommended values of the fundamental physical constants 2006,” Rev. Pachucki, “Muonic hydrogen and the proton radius puzzle,” Ann. Rev. Borie, “Lamb shift in light muonic atoms,” Ann. Jentschura, “Lamb shift in muonic hydrogen,” Ann. Pachucki, “Theory of the Lamb shift in muonic hydrogen,” Phys. Antognini et al., “Proton structure from the measurement of 2S-2P transition frequencies of muonic hydrogen,” Science. ![]()
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