Experiments towards resolving the proton charge radius puzzle

Abstract

Abstract.We review the status of the proton charge radius puzzle. Emphasis is given to the various experiments initiated to resolve the conflict between the muonic hydrogen results and the results from scattering and regular hydrogen spectroscopy. The proton charge radius puzzleThe historical route to the proton charge radius (r p ) is from elastic electron-proton scattering. In a completely complementary fashion, it has been obtained also from "high-precision" laser spectroscopy of hydrogen (H). Since a few years, "high-sensitivity" laser spectroscopy of muonic hydrogen (μp) offers a third way. The value extracted from μp with a relative accuracy of 5×10 −4 is an order of magnitude more accurate than obtained from the other methods. Yet the value is 4% smaller than derived from electron-proton scattering and H spectroscopy with a disagreement at the 7σ level [1][2][3][4][5].In the last five years as summarized in [6,7] various cross checks and refinements of boundstate QED calculations needed for the extraction of r p from μp have been performed, together with investigations of the proton structure. Several suggestions in the field of "beyond standard model" BSM physics have been articulated, re-analysis of scattering data have been carried out and new experiments have been initiated. Despite this, presently the discrepancy still persists and the resolution a

Confinement in Nuclei and the Expanding Proton

“…For example, [8] measures isotope shifts in the radii of Ca isotopes to better than 1% accuracy. New muonic atom measurements [9] that determine the charge radii of light nuclei are now at about the 1% level. Furthermore, a current Jefferson Laboratory experiment [10,11] aims to measure the the difference between the charge radii of 3 He and 3 H to a precision of ±0.02 fm.…”

High-accuracy deep-UV Ramsey-comb spectroscopy in krypton

“…Each laser pulse then sees a fresh gas sample from the jet, and therefore a constant phase shift in the HHG process. Such a measurement could provide important new data to the proton radius puzzle, as the transition can then be compared to spectroscopy of muonic He + ions [13]. …”

“…Both can be obtained from spectroscopy of atomic hydrogen, assuming that QED is sufficiently precise. However, when the CREMA collaboration determined the proton charge radius from spectroscopy in muonic hydrogen (consisting of a proton and a muon), it leads to a considerable (7σ) mismatch with the value extracted from normal (electronic) hydrogen [11][12][13]. This mismatch, known as the proton radius puzzle, remains to be explained and requires more spectroscopic measurements, e.g., in systems other than (muonic) hydrogen.…”

“…One example is molecular hydrogen, made possible by recent improvements in theory [15]. Another is He + [16], which can be compared to muonic-He + spectroscopy [13]. The experimental challenge is the short wavelengths required for excitation, which ranges from the deep UV (≈ 200 nm) for H 2 to extreme ultraviolet (XUV, < 60 nm) for He + .…”

Review of experimental and theoretical status of the proton radius puzzle

“…For an overview of other potential regular hydrogen measurements see Ref. [79]. A range of new measurements with hydrogen-like He + ions [80][81][82], molecules and molecular ions [83][84][85][86][87], and circular Rydberg states [88,89] are also anticipated.…”