Precise laser spectroscopy of the antiprotonic helium atom and CPT test on antiproton mass and charge

Abstract We have measured twelve transition frequencies of the antiprotonic helium atom (pHe + ) with precisions of 0.1–0.2 ppm using a laser spectroscopic method. The agreement between the experiment and theories was so good that we can put a limit on the proton-antiproton mass (or charge) difference. The new limit is expected to be much smaller than the already published value, 60 ppb (6 × 10 −8 ).