Charge Radius Experiment with Muonic Atoms

What is the proton size ?

The proton

Principle of the experiment on muonic hydrogen

Experimental set-up

Results

Muonic helium spectroscopy

The key point of this experiment is the measurement of the 2S-2P Lamb shift in muonic hydrogen (an hydrogen atom in which a muon takes the place of the electron). As the muon is 207 times heavier than the electron, muonic hydrogen has a correspondingly smaller size than a hydrogen atom. The relative contribution of the proton charge to the Lamb shift is as much as 2%, that is two orders of magnitude more than for normal hydrogen. For the same reason, the effect of vacuum polarization is more important than the self-energy. Consequently the 2S level is below the 2P one and the 2S-2P transition is in the infrared region, around 6 µm. The transition probability is smaller by a factor (about ) compares to normal hydrogen. This very weak transition probability, added to the small number of muonic hydrogen atoms one can product, makes this experiment very difficult. The signal is only a few events per hour…