The size of the proton
Dr. Randolf Pohl (MPI für Quantenoptik, Garching)

The charge radius Rp of the proton has so far been known with a precision of about 1% from both electron scattering and precision spectroscopy of hydrogen.

We have recently determined Rp by means of laser spectroscopy of the exotic "muonic hydrogen" atom [1]. Here, the muon, which is the 200 times heavier cousin of the electron, orbits the proton with a 200 times smaller Bohr radius. This enhances the sensitivity to the proton's finite size tremendously.

Our new value Rp = 0.84184 (67) fm is ten times more precise than the generally accepted CODATA value, but it differs by 5 standard deviations from it. A lively discussion about possible solutions to the "proton size puzzle" has started.
[1] R. Pohl et al., "The size of the proton", Nature 466, 213 (July 2010)