**Background**: There is significant current interest in knowing the value of the proton radius and also its proper definition.**Purpose**: Combine the disparate literatures of hydrogen spectroscopy and diverse modern parton distributions to understand the meaning of the proton radius in a manner consistent with the separate bodies of work. Methods: Use perturbation theory, light-front dynamics, and elementary techniques to find relativistically correct definitions of the proton radius and charge density.**Results**: It is found that the very same proton radius is accessed by measurements of hydrogen spectroscopy and elastic lepton scattering. The derivation of the mean-square radius as a moment of a spherically symmetric three dimensional density is shown to be incorrect. A relativistically correct, two-dimensional charge density is related to the diverse modern literature of various parton distributions. Relativistically invariant moments thereof are derived in a new relativistic moment expansion, the RME.**Conclusion**: The equation r p 2 ≡ −6G’ E (0) is the definition of the proton radius.

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# Defining the proton radius: A unified treatment

Gerald Miller, University of Washington

Thursday, April 25, 2019 - 3:45pm

NPL 178