Just over a century ago, by providing an explanation of atomic emission spectra, Bohr's model of the hydrogen atom helped lay the foundations of quantum mechanics. Today, probing energy levels of atomic systems remains an important tool in further developing our understanding of fundamental atomic properties.
Randolph Pohl and colleagues from the Charge Radius Experiment with Muonic Atoms (CREMA) collaboration have now performed laser spectroscopy measurements on muonic deuterium, an exotic atom with a nucleus consisting of one proton and one neutron — a deuteron — orbited by a muon. They prepared atoms in the metastable 2S state, and subsequently measured the frequencies of 2S–2P transitions.
The resulting values enabled a precise determination of the root-mean-square charge radius of the deuteron, which is linked to the value of the Rydberg constant. At 2.12562(78) × 10−15 m, this radius is significantly smaller than the conventional value obtained from electron scattering experiments. Together with similar results obtained earlier for muonic hydrogen, the findings of Pohl et al. imply a smaller proton radius, posing a serious challenge for the standard model of particle physics.