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Details

Autor(en) / Beteiligte
Titel
A small proton charge radius from an electron–proton scattering experiment
Ist Teil von
  • Nature (London), 2019-11, Vol.575 (7781), p.147-150
Ort / Verlag
London: Nature Publishing Group UK
Erscheinungsjahr
2019
Beschreibungen/Notizen
  • Elastic electron–proton scattering (e–p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r p . In 2010, a new method using muonic hydrogen atoms 1 found a substantial discrepancy compared with previous results 2 , which became known as the ‘proton radius puzzle’. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen 3 , 4 . Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e–p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e–p experiments and enabled measurements at very small forward-scattering angles. Our result, r p  = 0.831 ± 0.007 stat  ± 0.012 syst  femtometres, is smaller than the most recent high-precision e–p measurement 5 and 2.7 standard deviations smaller than the average of all e–p experimental results 6 . The smaller r p we have now measured supports the value found by two previous muonic hydrogen experiments 1 , 7 . In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant 8 —one of the most accurately evaluated fundamental constants in physics. A magnetic-spectrometer-free method for electron–proton scattering data reveals a proton charge radius 2.7 standard deviations smaller than the currently accepted value from electron–proton scattering, yet consistent with other recent experiments.

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