Proton-electron mass ratio from laser spectroscopy of HD+ at the part-per-trillion level

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Science  04 Sep 2020:
Vol. 369, Issue 6508, pp. 1238-1241
DOI: 10.1126/science.aba0453

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A very precise ratio

The value of the ratio of the masses of the proton and the electron has a bearing on the values of other physical constants. This ratio is known to a very high precision. Patra et al. improved this precision even further by measuring particular frequencies in the rovibrational spectrum of the hydrogen deuteride molecular ion (HD+) (see the Perspective by Hori). To reach this high precision, the researchers placed the HD+ molecules in an ion trap and surrounded them by beryllium ions. The cold beryllium ions then helped cool the HD+ molecules, making the HD+ spectral lines narrow enough that the proton-electron mass ratio could be extracted by comparison with theoretical predictions.

Science, this issue p. 1238; see also p. 1160


Recent mass measurements of light atomic nuclei in Penning traps have indicated possible inconsistencies in closely related physical constants such as the proton-electron and deuteron-proton mass ratios. These quantities also influence the predicted vibrational spectrum of the deuterated molecular hydrogen ion (HD+) in its electronic ground state. We used Doppler-free two-photon laser spectroscopy to measure the frequency of the v = 0→9 overtone transition (v, vibrational quantum number) of this spectrum with an uncertainty of 2.9 parts per trillion. By leveraging high-precision ab initio calculations, we converted our measurement to tight constraints on the proton-electron and deuteron-proton mass ratios, consistent with the most recent Penning trap determinations of these quantities. This results in a precision of 21 parts per trillion for the value of the proton-electron mass ratio.

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