The usual measure of the size of a proton is its charge radius, which can be determined from spectroscopy of the hydrogen atom, using the Lamb shift and the fact that s and p waves behave differently near the origin. Now, Randolf Pohl of the Max Planck Institute for Quantum Optics in Garching and colleagues have used pulsed-laser spectroscopy to perform the analogous measurement for muonic hydrogen, where a negative muon takes the place of the usual electron.
The result is a proton radius that is 10 times more precise than the current world average (mainly obtained from ordinary hydrogen) – but, surprisingly, five standard deviations smaller. It is now a challenge to theorists to explain this result and to experimentalists to see if some error has been made.