Abstract
The proton radius puzzle is the disagreement between the much more precise radius determined from muonic hydrogen spectroscopy and the numerous atomic hydrogen and electron scattering determinations. The puzzle has several possible resolutions, including physics beyond the Standard Model, missing conventional physics, and errors or underestimated uncertainties in the extraction of the radius from the data. New experiments are needed to confirm and / or resolve the puzzle. The MU on S cattering E xperiment (MUSE) recently approved at PSI has been designed to help resolve the puzzle by measuring the radius in a way not yet done. Similar to electron scattering, the radius will be extracted from the observed change of the charge form factor with momentum transfer. The experiment uses the π M1 beamline to provide a mixed secondary muon and electron (and pion) beam of either positive or negative charge. The comparison of muon and electron scattering measured simultaneously determines the consistency of the form factors in the two cases with high precision. Comparison of yields from both charge signs will at the same time disentangle the effect of two-photon exchange. The proton charge radius can be extracted from each set of scattering data. The physics case and status of MUSE will be discussed.
Highlights
The proton charge radius has been extracted from the Lamb shift of 2S-2P transitions in regular hydrogen in the framework of bound-state Quantum ae-mail: kohlm@jlab.org
The CODATA2010 value [7] of rp = (0.8775 ± 0.0051) fm is based on electron scattering and Lamb shift measurements from regular hydrogen
The biggest possible effects due to conventional physics are related to the muon vacuum polarization [9], which have been questioned [10], and due to the proton polarizability contribution to the effect of two-photon exchange, which could be different in size for ep and μp interaction [11,12,13,14,15]
Summary
The proton rms radius has been measured with elastic electron-proton scattering since the 1950’s by investigating how the form factor changes near Q2 = 0. The proton rms charge radius contributes to the Lamb shift of the 2S-2P atomic transitions of the hydrogen atom, due to the overlap of the atomic shells with the finite-sized nuclear center. The proton charge radius has been extracted from the Lamb shift of 2S-2P transitions in regular hydrogen in the framework of bound-state Quantum ae-mail: kohlm@jlab.org
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