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

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