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The proton radius puzzle

New results from an experiment at the Paul Scherrer Institute (PSI) in Switzerland which measure the Lamb shift of muonic hydrogen (a hydrogen atom where the electron has been replaced by a muon - a heavier version of the electron) allow an accurate calculation of the proton radius. Other techniques, namely, Lamb shift measurement of electronic hydrogen and measurements of the proton form factors in electron scattering also allow an extraction of the radius. Surprisingly, the new, highly accurate results from PSI disagree with the current best world results by a huge factor (~6 standard deviations), see the figure below (Paul et al. is the PSI result, “this work” is a result from our recent JLab experiment).



Together with several other groups we are considering a new set of experiments which may help resolve this puzzle. Most prominent among these is an idea to perform an analog of the electron scattering experiments using the muon beam at PSI. This effort is led by the Rutgers group, but we expect to play a leading role in any future experiment. The muon beam experiment has passed a preliminary technical review and we have just taken some test beam at PSI to examine the properties of the muon beam. The experiment has been approved in January 2013 and has already undergone two successful test runs, designed to test various aspects of the beam line and detector configurations. We expect funding during 2014 with construction starting shortly thereafter.

See more here.


References:

  • Pohl et al., The Size of the Proton, Nature 466, 213 (2010).
  • A. Antognin et al., Proton Structure from the Measurement of 2S−2P Transition Frequencies of Muonic Hydrogen, Science 339, 417 (2013).
  • Muonic Hydrogen and the Proton Radius Puzzle, R. Pohl et al., arxiv:1301.0905.
  • X. Zhan et al., High Precision Measurement of the Proton Elastic Form Factor Ratio μpGE/GM at low Q2, Phys. Lett. B705, 59 (2011).