Hamish Johnston in Physics World:
The radius of the proton is significantly smaller than previously thought, say physicists who have measured it to the best accuracy yet. The surprising result was obtained by studying “muonic” hydrogen in which the electron is replaced by a much heavier muon. The finding could mean that physicists need to rethink how they apply the theory of quantum electrodynamics (QED) – or even that the theory itself needs a major overhaul.
A proton contains three charged quarks bound by the strong force and its radius is defined as the distance at which the charge density drops below a certain value. The radius has been measured in two main ways – by scattering electrons from hydrogen and by looking very closely at the difference between certain energy levels of the hydrogen atom called the Lamb shift. Until recently the best estimate of the proton radius was 0.877 femtometres with an uncertainty of 0.007 fm
This Lamb shift is a result of the interactions between the electron and the constituent quarks of the proton as described by QED. These interactions are slightly different for electrons occupying the 2S and 2P energy levels and the resulting energy shift depends in part on the radius of the proton.
However, in muonic hydrogen the Lamb shift is much more dependent on the proton radius because the much heavier muon spends more time very near to – and often within – the proton itself.
Now an international team led by Randolf Pohl at the Max Planck Institute for Quantum Optics in Garching, Germany has measured the Lamb shift in muonic hydrogen for the first time and found the proton radius to be 0.8418 fm with uncertainty 0.0007 fm.