News from the muonic helium lamb-shift experiment

Our ongoing experiment located at Paul-Scherrer-Institute (Switzerland) recently succeeded to measure the 2S_1/2-2P_3/2 transition in the muonic Helium-4-ion, and will continue to measure the remaining 2S-2P transitions in μ4He⁺ and μ3He⁺ later this summer. Due to its sensitivity to finite size effects, the Lamb-shift in muonic atoms is an excellent tool to determine nuclear rms charge radii, important input parameters in both nuclear models and atomic theory. With our result, we will be able to provide a ten times more accurate value for the absolute nuclear charge radius of the alpha particle, together with the respective 3He, 6He and 8He values that can be extracted via already measured isotope shifts. Furthermore, our data sheds interesting new light on the so-called ''proton size puzzle'', created by the 7-sigma discrepancy between the muonic hydrogen value of the proton radius and other experiments.