Laser spectroscopy of muonic hydrogen: New determination of the proton charge radius

We report on the first laser spectroscopy experiment on muonic hydrogen. We have measured the 2S-2P Lamb shift in muonic hydrogen. A new and unexpected value of the proton root-mean-square charge radius (r_p) is deduced from this measurement. This value of r_p strongly disagrees with the others determinations either from hydrogen spectroscopy combined with QED calculations or from electron scattering experiments. Our result provides a completely new picture of the comparison between transition energies measurements and QED calculations. This experiment has been realized at Paul Scherrer Institute (Villigen Switzerland) in the frame of the international collaboration CREMA (see references and http://muhy.web.psi.ch ).