Gil Paz (Wayne State U.)

The Charge Radius of the Proton, a Five Sigma Discrepancy?

The charge radius of the proton is a basic non-perturbative parameter.

Recently, it was extracted for the first time from the Lamb shift in

muonic hydrogen. For a long time it was anticipated that such a

measurement would reduce the error by an order of magnitude compared to

measurements from electron- proton scattering and regular hydrogen

spectroscopy. While this goal was achieved, the value of the proton¹s

charge radius that was obtained was, very surprisingly, five standard

deviations away from the world average.

The extraction of the charge radius from the Lamb shift in muonic

hydrogen depends on a theoretical input. Together with Richard J. Hill,

we are studying the hadronic uncertainty in the theoretical prediction

using the tool of an effective field theory, namely NRQED. In the

talk I will describe the results of this study. I will also describe a

previous study of the model-independent extraction of the charge radius

from electron-proton scattering We have shown that previous extractions,

spanning a period of over 40 years, have underestimated their error

sometimes by a factor of two or more.

Feb 13 at 4pm in Geo/Phys Room 407