Study of the hyperfine structure of antiprotonic helium

The metastable states of antiprotonic helium have a unique magnetic substructure called hyperfine structure, arising from the coupling of the large angular momentum of the antiproton (l∼35) with the electron and antiproton spins. We designed, developed and performed a laser-microwave-laser resonance spectroscopy experiment to investigate this hyperfine structure. We observed two microwave transitions, and the measured frequencies agree well with recent theoretical calculations. To investigate possible collision-induced shifts of the transition frequencies, we measured the microwave transition frequencies at two different target densities. Within the limited statistics, we did not observe a clear sign of a density shift, in accordance with a theoretical estimate by Korenman. When averaging the transition frequencies over the density, we reach an agreement of about 60 ppm with the three-body QED calculations, which is of the same order than the accuracy of the calculations, and slightly larger than the experimental precision