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

In 2009 we observed two 2S-2P transitions of muonic hydrogen ; they are reported on the level scheme below. The largest signal is the one associated to the transition from the 2S(F=1) level. From the two resonance positions, we can deduce both the n=2 Lamb shift and the hyperfine structure of the 2S level. The respective values obtained are 202.3706(23) meV and 22.8089(51) meV.

By comparing the measured Lamb shift with its theoretical expression depending on the proton charge radius, a determination of this radius is derived : = 0.84087(39) fm

The hyperfine structure allows to deduce the proton Zemach radius : = 1.082(37) fm

This value is in good agreement with the ones obtained by other methods.

From the proton charge radius and Zemach radius, one can derive the proton magnetic radius (associated to its magnetic moment distribution) : = 0.87 (6) fm

The value given above is one order of magnitude more precise than the 2010-CODATA value, deduced from spectroscopy of hydrogen atom (e-p) and electron-proton diffusion experiments, but deviates from it by 7 sigma.

These results published in Science (2013) reinforce the proton radius puzzle already pointed out in Nature (2010).