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Proton Charge Radius from Electron Scattering

Department of Physics, University of Basel, Basel CH4056, Switzerland
Received: 21 November 2017 / Revised: 19 December 2017 / Accepted: 20 December 2017 / Published: 30 December 2017
(This article belongs to the Special Issue High Precision Measurements of Fundamental Constants)
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The rms-radius R of the proton charge distribution is a fundamental quantity needed for precision physics. This radius, traditionally determined from elastic electron-proton scattering via the slope of the Sachs form factor G e ( q 2 ) extrapolated to momentum transfer q 2 = 0 , shows a large scatter. We discuss the approaches used to analyze the e-p data, partly redo these analyses in order to identify the sources of the discrepancies and explore alternative parameterizations. The problem lies in the model dependence of the parameterized G ( q ) needed for the extrapolation. This shape of G ( q < q m i n ) is closely related to the shape of the charge density ρ ( r ) at large radii r, a quantity that is ignored in most analyses. When using our physics knowledge about this large-r density together with the information contained in the high-q data, the model dependence of the extrapolation is reduced, and different parameterizations of the pre-2010 data yield a consistent value for R = 0.887 ± 0.012 fm. This value disagrees with the more precise value 0.8409 ± 0.0004 fm determined from the Lamb shift in muonic hydrogen. View Full-Text
Keywords: proton rms-radius; electron scattering proton rms-radius; electron scattering

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Sick, I. Proton Charge Radius from Electron Scattering. Atoms 2018, 6, 2.

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