Is the proton radius a player in the redefinition of the International System of Units?

Abstract

International audienceIt is now recognized that the International System of Units (SI units) will be redefined in terms of fundamental constants, even if the date when this will occur is still under debate. Actually, the best estimate of fundamental constant values is given by a least-squares adjustment, carried out under the auspices of the Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants. This adjustment provides a significant measure of the correctness and overall consistency of the basic theories and experimental methods of physics using the values of the constants obtained from widely differing experiments. The physical theories that underlie this adjustment are assumed to be valid, such as quantum electrodynamics (QED). Testing QED, one of the most precise theories is the aim of many accurate experiments. The calculations and the corresponding experiments can be carried out either on a boundless system, such as the electron magnetic moment anomaly, or on a bound system, such as atomic hydrogen. The value of fundamental constants can be deduced from the comparison of theory and experiment. For example, using QED calculations, the value of the fine structure constant given by the CODATA is mainly inferred from the measurement of the electron magnetic moment anomaly carried out by Gabrielse's group. (Hanneke et al. 2008 Phys. Rev. Lett. 100, 120801) The value of the Rydberg constant is known from two-photon spectroscopy of hydrogen combined with accurate theoretical quantities. The Rydberg constant, determined by the comparison of theory and experiment using atomic hydrogen, is known with a relative uncertainty of 6.6 x 10(-12). It is one of the most accurate fundamental constants to date. A careful analysis shows that knowledge of the electrical size of the proton is nowadays a limitation in this comparison. The aim of muonic hydrogen spectroscopy was to obtain an accurate value of the proton charge radius. However, the value deduced from this experiment contradicts other less accurate determinations. This problem is known as the proton radius puzzle. This new determination of the proton radius may affect the value of the Rydberg constant R-infinity. This constant is related to many fundamental constants; in particular, R-infinity links the two possible ways proposed for the redefinition of the kilogram, the Avogadro constant N-A and the Planck constant h. However, the current relative uncertainty on the experimental determinations of N-A or h is three orders of magnitude larger than the 'possible' shift of the Rydberg constant, which may be shown by the new value of the size of the proton radius determined from muonic hydrogen. The proton radius puzzle will not interfere in the redefinition of the kilogram. After a short introduction to the properties of the proton, we will describe the muonic hydrogen experiment. There is intense theoretical activity as a result of our observation. A brief summary of possible theoretical explanations at the date of writing of the pap...

When should we change the definition of the second?

“…The difficulty with this is loss of accuracy of the 1S-2S transition relative to current Cs performance, and that between the measured value of 1S-2S and the Rydberg constant, which arises due to uncertainties in QED theory corrections and knowledge of the proton size. This latter issue is the subject of the paper by Nez [23] in this issue.…”

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“…The Rydberg constant is considered to be one of the most well-determined physical constants, with an accuracy of 7 parts to 10 12 and is thus used to constrain the other physical constants [45]. However, as the same spectroscopic experiments are used to determine both the charge radius of the proton and the Rydberg constant, the recent muonic measurements of the charge radius of the proton implies that the Rydberg constant would change by 45  − [46] [47] [48]. This is known as the proton radius puzzle.…”

Highly Charged Ions in Rare Earth Permanent Magnet Penning Traps

“…This offers a new avenue for the deter-mination of fundamental constants, such as the Rydberg constant, if precise optical measurements can be realized to test theory. Such tests could potentially be useful in making improved measurements of the constants of nature that form the basis of modern metrology [18,17]-efforts that are more compelling in the wake of the large discrepancy in proton radius determinations that resulted from recent muonic hydrogen Lamb-shift measurements [22,20].…”